Fixed #5937 (ValueFlow: wrong analysis of calculations with different variable operands)
Fixed #5959 (ValueFlow: return value from subfunction)
Print callstack in SEH on Windows
Fix cppcheck warning in previous changeset

Signed-off-by: amai2012 <amai@users.sf.net>
astyle formatting

[ci skip]
valueFlowBefore: better analysis of conditional assignment
Fixed #5939 (fp: Possible null pointer dereference, after check against NULL in for loop)
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: add -DCHECK_INTERNAL to the VERIFY=1 job to automatically fetch issues like #5869 via travis, check clang with --exception-handling
travis: make clang check sensitive to internal cppcheck errors.