Deduplicate offset calculation in QMetaObjectBuilder

Not even an optimization (unless your compiler sucks). Just improves
readability, since offset is already calculated few lines above.

Offset formula used to be somewhat more complicated, but since commit
e58b44d557 it became reusable here.

Change-Id: I4128b2643daf2cb112c9b861342fd0e27c9a1a95
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qmetaobjectbuilder.cpp

@@ -1119,7 +1119,7 @@ static void writeString(char *out, int i, const QByteArray &str,
     memcpy(out + 2 * i * sizeof(uint), &offsetLen, 2 * sizeof(uint));
 
     memcpy(out + offset, str.constData(), size);
-    out[offsetOfStringdataMember + stringdataOffset + size] = '\0';
+    out[offset + size] = '\0';
 
     stringdataOffset += size + 1;
 }