[analyzer] Fix an assertion fail in hash generation.

[xazax.hun]

Summary

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.

This error was reproduced by an internal checker that warned when the header guard is missing in a header. The location of the warning was the sart location of the header file which might only contain whitespaces in some cases.

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 easy to wrote a custom debug checker for that purpuse but I think it might not worth the effort.

[zaks.anna]

LGTM.

cfe/trunk/lib/StaticAnalyzer/Core/IssueHash.cpp

 }
 
 static std::string NormalizeLine(const SourceManager &SM, FullSourceLoc &L,
                                  const LangOptions &LangOpts) {
   static StringRef Whitespaces = " \t\n";
 
   StringRef Str = GetNthLineOfFile(SM.getBuffer(L.getFileID(), L),
                                    L.getExpansionLineNumber());
-  unsigned col = Str.find_first_not_of(Whitespaces);
+  StringRef::size_type col = Str.find_first_not_of(Whitespaces);
+  if (col == StringRef::npos)
+    col = 1; // The line only contains whitespace.
+  else
     col++;
   SourceLocation StartOfLine =
       SM.translateLineCol(SM.getFileID(L), L.getExpansionLineNumber(), col);
   llvm::MemoryBuffer *Buffer =
       SM.getBuffer(SM.getFileID(StartOfLine), StartOfLine);
   if (!Buffer)
     return {};
 
   const char *BufferPos = SM.getCharacterData(StartOfLine);