Fix compilation

Commit be491913c0 regressed the build with some
compilers that complained that the RegExp fields were uninitialized. Fix is
the same as for the Lookup class.

Change-Id: Ie6b2adf14496d75845f6a9d9b8fc680e61aa1155
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

src/qml/compiler/qv4compileddata_p.h

@@ -146,6 +146,8 @@ struct RegExp
         QJsonPrivate::qle_bitfield<0, 4> flags;
         QJsonPrivate::qle_bitfield<4, 28> stringIndex;
     };
+
+    RegExp() { flags = 0; stringIndex = 0; }
 };
 
 struct Lookup