storagefile: qcow1: Fix check for empty backing file

From f772b3d91f the intention of this code seems to be to set
format=NONE when the image does not have a backing file. However
'buf' here is the whole qcow1 file header. What we want to be
checking is 'res' which is the parsed backing file path.
qcowXGetBackingStore sets this to NULL when there's no backing file.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>

src/util/virstoragefile.c

@@ -578,7 +578,7 @@ qcow1GetBackingStore(char **res,
      * used to store backing format */
     *format = VIR_STORAGE_FILE_AUTO;
     ret = qcowXGetBackingStore(res, NULL, buf, buf_size, false);
-    if (ret == BACKING_STORE_OK && *buf == '\0')
+    if (ret == BACKING_STORE_OK && !*res)
         *format = VIR_STORAGE_FILE_NONE;
     return ret;
 }