#5875 clang ubsan errors: lib/checkassignif.cpp:58:34: runtime error: member call on null pointer of type 'Token'
TokenList: added assertion that makes sure the Preprocessor doesn't let through extended ascii wrongly.
#5875 Fix 'lib/mathlib.cpp:559:43: runtime error: division by zero'. Several fixes to MathLib::divide(): avoid division by zero, handle various representation of 0 value, use 'nan.' instead of '-nan.0'
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"
travis: add -DCHECK_INTERNAL to the VERIFY=1 job to automatically fetch issues like #5869 via travis