Fix buffer underrun (invalid read) triggered during diagnostic rendering. The test would overflow when computing '0 - 1'.

I don't have a good testcase for this that does not depend on system headers.
It did not trigger with preprocessed output, and I had trouble reducing the example.

Fixes <rdar://problem/13324594>.

Thanks to Michael Greiner for reporting this issue.

llvm-svn: 177201

clang/lib/Frontend/TextDiagnostic.cpp

@@ -958,7 +958,7 @@ static void highlightRange(const CharSourceRange &R,
     // Pick the last non-whitespace column.
     if (EndColNo > map.getSourceLine().size())
       EndColNo = map.getSourceLine().size();
-    while (EndColNo-1 &&
+    while (EndColNo &&
            (map.getSourceLine()[EndColNo-1] == ' ' ||
             map.getSourceLine()[EndColNo-1] == '\t'))
       EndColNo = map.startOfPreviousColumn(EndColNo);