GUI: Minor image compression improvements.

Before: 24.321 bytes
After:  24.272 bytes

[ci skip]
Fixed CID 1216439: potential copy&paste error in testing symbol database.
cleanup redundant valueflow check. if there are no values the getValue will return NULL.
Fix missing break while handling SIGBUS - fixes clang compiler warning
Ticket #5615: Avoid calling the same function n times when once is enough.
Merge pull request #326 from simartin/ticket_5615_avoid_useless_similar_calls

Ticket #5615: Avoid calling the same function n times when once is enough
travis: replace kernel check with clang check.

Check clang instead of kernel, which is a lot faster.
Make clang check failure fatal to the build.
travis: run test_matchcompiler.py
travis: move CHECK_INTERNAL check up, remove "SRCDIR=build VERIFY=1" job in favor of "MAKEFLAGS="HAVE_RULES=yes" SRCDIR=build VERIFY=1"
add -DCHECK_INTERNAL to the VERIFY=1 job to automatically fetch issues like #5869 via travis.