-D parsing in psscale (colorbar) may fail to check if refpoint is set

[Chuan1937]

When the -D parameter of psscale uses an invalid alignment code (e.g., -DJ+e), the gmt_just_decode function returns -99. However, gmt_get_refpoint passes this return value directly to subsequent code without checking it, resulting in the mysterious warning gmt_flip_justify called with incorrect argument (-99).

The fix involves checking immediately after calling gmt_just_decode whether the return value is -99. If so, it outputs the clear error message Invalid justification code
and returns NULL, thereby providing users with explicit error guidance.

Fixes #8864

Local test successful:

      Start 847: test/psscale/autoscale.sh
 1/29 Test #847: test/psscale/autoscale.sh .........   Passed    1.03 sec
      Start 848: test/psscale/categoricalcpt.sh
 2/29 Test #848: test/psscale/categoricalcpt.sh ....   Passed    0.76 sec
      Start 849: test/psscale/catlabels.sh
 3/29 Test #849: test/psscale/catlabels.sh .........   Passed    1.29 sec
      Start 850: test/psscale/cbarfontsize.sh
 4/29 Test #850: test/psscale/cbarfontsize.sh ......   Passed    0.86 sec
      Start 851: test/psscale/cbars.sh
 5/29 Test #851: test/psscale/cbars.sh .............   Passed    0.96 sec
      Start 852: test/psscale/colorbar-nans-h.sh
 6/29 Test #852: test/psscale/colorbar-nans-h.sh ...   Passed    1.18 sec
      Start 853: test/psscale/colorbar-nans-v.sh
 7/29 Test #853: test/psscale/colorbar-nans-v.sh ...   Passed    1.29 sec
      Start 854: test/psscale/cpthinges.sh
 8/29 Test #854: test/psscale/cpthinges.sh .........   Passed    1.85 sec
      Start 855: test/psscale/cptmix.sh
 9/29 Test #855: test/psscale/cptmix.sh ............   Passed    0.95 sec
      Start 856: test/psscale/cpttime.sh
10/29 Test #856: test/psscale/cpttime.sh ...........   Passed    0.59 sec
      Start 857: test/psscale/customwidths.sh
11/29 Test #857: test/psscale/customwidths.sh ......   Passed    0.85 sec
      Start 858: test/psscale/cyclecpt.sh
12/29 Test #858: test/psscale/cyclecpt.sh ..........   Passed    0.96 sec
      Start 859: test/psscale/hardsoft.sh
13/29 Test #859: test/psscale/hardsoft.sh ..........   Passed    1.88 sec
      Start 860: test/psscale/horizontal.sh
14/29 Test #860: test/psscale/horizontal.sh ........   Passed    1.47 sec
      Start 861: test/psscale/horsclpanel.sh
15/29 Test #861: test/psscale/horsclpanel.sh .......   Passed    1.09 sec
      Start 862: test/psscale/interval-panel.sh
16/29 Test #862: test/psscale/interval-panel.sh ....   Passed    1.03 sec
      Start 863: test/psscale/pattern.sh
17/29 Test #863: test/psscale/pattern.sh ...........   Passed    0.97 sec
      Start 864: test/psscale/placebars.sh
18/29 Test #864: test/psscale/placebars.sh .........   Passed    1.48 sec
      Start 865: test/psscale/psscale-l2s.sh
19/29 Test #865: test/psscale/psscale-l2s.sh .......   Passed    0.92 sec
      Start 866: test/psscale/psscale.sh
20/29 Test #866: test/psscale/psscale.sh ...........   Passed    1.58 sec
      Start 867: test/psscale/scale_percent.sh
21/29 Test #867: test/psscale/scale_percent.sh .....   Passed    1.01 sec
      Start 868: test/psscale/scales.sh
22/29 Test #868: test/psscale/scales.sh ............   Passed    1.04 sec
      Start 869: test/psscale/scalings.sh
23/29 Test #869: test/psscale/scalings.sh ..........   Passed    1.76 sec
      Start 870: test/psscale/seis.sh
24/29 Test #870: test/psscale/seis.sh ..............   Passed    1.43 sec
      Start 871: test/psscale/slant_annot.sh
25/29 Test #871: test/psscale/slant_annot.sh .......   Passed    1.43 sec
      Start 872: test/psscale/transp_scale.sh
26/29 Test #872: test/psscale/transp_scale.sh ......   Passed    1.15 sec
      Start 873: test/psscale/versclpanel.sh
27/29 Test #873: test/psscale/versclpanel.sh .......   Passed    1.26 sec
      Start 874: test/psscale/vertcustannot.sh
28/29 Test #874: test/psscale/vertcustannot.sh .....   Passed    1.00 sec
      Start 875: test/psscale/vertical.sh
29/29 Test #875: test/psscale/vertical.sh ..........   Passed    1.37 sec

100% tests passed, 0 tests failed out of 29

Total Test time (real) =  34.49 sec