qemu: Resolve Coverity FORWARD_NULL

If we end up at the cleanup lable before we've VIR_EXPAND_N the list,
then calling virQEMUCapsFreeStringList() with a NULL proplist could
theoretically deref proplist if nproplist was set. Coverity doesn't
seem to acknowledge the relationship between proplist and nproplist
assuming in virQEMUCapsFreeStringList that nproplist could be at
least 1 and thus have a null deref.  It only seems to follow the
NULL proplist.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_capabilities.c

@@ -1728,7 +1728,7 @@ virQEMUCapsParseDeviceStrObjectProps(const char *str,
     ret = nproplist;
 
  cleanup:
-    if (ret < 0)
+    if (ret < 0 && proplist)
         virQEMUCapsFreeStringList(nproplist, proplist);
     return ret;
 }