Lex: Don't crash if both conflict markers are on the same line

We would check if the terminator marker is on a newline.  However, the
logic would end up out-of-bounds if the terminator marker immediately
follows the start marker.

This fixes PR21820.

llvm-svn: 224210

clang/lib/Lex/Lexer.cpp

@@ -2588,8 +2588,8 @@ static const char *FindConflictEnd(const char *CurPtr, const char *BufferEnd,
   size_t Pos = RestOfBuffer.find(Terminator);
   while (Pos != StringRef::npos) {
     // Must occur at start of line.
-    if (RestOfBuffer[Pos-1] != '\r' &&
-        RestOfBuffer[Pos-1] != '\n') {
+    if (Pos == 0 ||
+        (RestOfBuffer[Pos - 1] != '\r' && RestOfBuffer[Pos - 1] != '\n')) {
       RestOfBuffer = RestOfBuffer.substr(Pos+TermLen);
       Pos = RestOfBuffer.find(Terminator);
       continue;

clang/test/Lexer/conflict-marker.c

@@ -36,3 +36,5 @@ int foo() {
   y a = x;
   return x + a - z;
 }
+
+<<<<<<<>>>>>>> // expected-error {{expected identifier}}