[clang-tidy] Fix bug in modernize-loop-convert check.

http://reviews.llvm.org/D12186

Patch by Angel Garcia!

llvm-svn: 245561
diff --git a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
index ad2393a..f3dd154 100644
--- a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
@@ -337,8 +337,9 @@
                                     const LangOptions &LangOpts,
                                     SourceRange Range) {
   if (SourceMgr.getFileID(Range.getBegin()) !=
-      SourceMgr.getFileID(Range.getEnd()))
-    return nullptr;
+      SourceMgr.getFileID(Range.getEnd())) {
+    return StringRef(); // Empty string.
+  }
 
   return Lexer::getSourceText(CharSourceRange(Range, true), SourceMgr,
                               LangOpts);