Fix assertion in BreakableBlockComment (http://llvm.org/PR21916).

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6894

llvm-svn: 225628
diff --git a/clang/lib/Format/BreakableToken.cpp b/clang/lib/Format/BreakableToken.cpp
index 1bea0e5..26f1371 100644
--- a/clang/lib/Format/BreakableToken.cpp
+++ b/clang/lib/Format/BreakableToken.cpp
@@ -303,7 +303,8 @@
     StartOfLineColumn[i] += Decoration.size();
     Lines[i] = Lines[i].substr(Decoration.size());
     LeadingWhitespace[i] += Decoration.size();
-    IndentAtLineBreak = std::min<int>(IndentAtLineBreak, StartOfLineColumn[i]);
+    IndentAtLineBreak =
+        std::min<int>(IndentAtLineBreak, std::max(0, StartOfLineColumn[i]));
   }
   IndentAtLineBreak = std::max<unsigned>(IndentAtLineBreak, Decoration.size());
   DEBUG({