[analyzer] Fix an assertion fail in hash generation.

In case the (uniqueing) location of the diagnostic is in a line that only
contains whitespaces there was an assertion fail during issue hash generation.

Unfortunately I am unable to reproduce this error with the built in checkers, 
so no there is no failing test case with this patch. It would be possible to
write a debug checker for that purpuse but it does not worth the effort.

Differential Revision: http://reviews.llvm.org/D18210

llvm-svn: 264851