fixed segfault at gl_compose

[asafalg]

This fixes the issue #430 reported earlier.

[codecov]

Codecov Report

Merging #431 into next will increase coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             next     #431      +/-   ##
==========================================
+ Coverage   36.99%   37.10%   +0.11%     
==========================================
  Files          45       45              
  Lines        8572     8575       +3     
==========================================
+ Hits         3171     3182      +11     
+ Misses       5401     5393       -8     

Impacted Files	Coverage Δ
src/backend/gl/gl_common.c	0.00% <0.00%> (ø)
src/log.h	56.25% <0.00%> (-25.00%)	⬇️
src/common.h	30.76% <0.00%> (-7.70%)	⬇️
src/win.c	64.43% <0.00%> (-0.21%)	⬇️
src/event.c	58.06% <0.00%> (+0.53%)	⬆️
src/region.h	20.00% <0.00%> (+8.88%)	⬆️
src/x.h	68.42% <0.00%> (+15.78%)	⬆️
src/utils.h	73.68% <0.00%> (+52.63%)	⬆️

[yshui]

Seems like there could be a deeper underlying bug. I don't think gl_compose should be called with img == NULL

[carmelopellegrino]

If the function returns on line 530, it produces a memory leak of coord and indices. By just moving the whole if statement two lines above the memory leak is avoided.