Restore yylval initialization

Forward port commit b04b7d340f to do the
initialization in the original file, not in the generated one.

Change-Id: I070d9668c75be20c014835514db49d1a6518d590
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Michael Brasser <michael.brasser@live.com>

src/qml/parser/qqmljs.g

@@ -451,6 +451,7 @@ Parser::Parser(Engine *engine):
     location_stack(0),
     string_stack(0),
     program(0),
+    yylval(0),
     first_token(0),
     last_token(0)
 {

src/qml/parser/qqmljsparser.cpp

@@ -92,6 +92,7 @@ Parser::Parser(Engine *engine):
     location_stack(0),
     string_stack(0),
     program(0),
+    yylval(0),
     first_token(0),
     last_token(0)
 {