Fix #1147 by checking NULL pointers

[ShellCode33]

Please review carefully, I'm not familiar at all with dunst nor Wayland. This might not be the right way to fix #1147.

[codecov-commenter]

Codecov Report

Merging #1148 (42c8b84) into master (7bd8e6b) will decrease coverage by 0.07%. The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
- Coverage   65.96%   65.90%   -0.07%     
==========================================
  Files          46       46              
  Lines        7548     7555       +7     
==========================================
  Hits         4979     4979              
- Misses       2569     2576       +7     

Flag	Coverage Δ
unittests	65.90% <0.00%> (-0.07%)	:arrow_down:

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Impacted Files	Coverage Δ
src/draw.c	51.35% <0.00%> (-0.33%)	:arrow_down:
src/wayland/wl.c	0.00% <0.00%> (ø)

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[fwsmit]

The change seems sensible. Have you tested if this fixes the bug?

[ShellCode33]

The bug doesn't seem to occur anymore with that patch

EDIT: dunst has been running for more than 6 hours with no issue

[fwsmit]

Thanks! The patch looks good, so I'll merge it