Initialize v_ to placate GCC

v_ has a value assigned to it as part of ParseNext() which happens in
the constructor, but that’s not soon enough for GCC

example/lookaheadparser/lookaheadparser.cpp

@@ -78,7 +78,7 @@ protected:
     static const int parseFlags = kParseDefaultFlags | kParseInsituFlag;
 };
 
-LookaheadParserHandler::LookaheadParserHandler(char* str) : ss_(str) {
+LookaheadParserHandler::LookaheadParserHandler(char* str) : v_(), ss_(str) {
     r_.IterativeParseInit();
     ParseNext();
 }