qmake: fix UB in QMakeParser::putHashStr()

Found by UBSan:

  qmake/library/qmakeparser.cpp:278:33: runtime error: null pointer passed as argument 2, which is declared to never be null

Guard the call.

Change-Id: I99341ab439a511f366dae9344ddcc8727c33b9b6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

qmake/library/qmakeparser.cpp

@@ -280,7 +280,8 @@ void QMakeParser::putHashStr(ushort *&pTokPtr, const ushort *buf, uint len)
     *tokPtr++ = (ushort)hash;
     *tokPtr++ = (ushort)(hash >> 16);
     *tokPtr++ = (ushort)len;
-    memcpy(tokPtr, buf, len * 2);
+    if (len) // buf may be nullptr; don't pass that to memcpy (-> undefined behavior)
+        memcpy(tokPtr, buf, len * 2);
     pTokPtr = tokPtr + len;
 }