block: qcow: Test rejection of backing file size with zero offset

Cover the malformed header case where backing_file_offset is zero
but backing_file_size is non zero, which must be rejected with the
new dedicated error.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/qcow/internal/mod.rs

@@ -2753,6 +2753,12 @@ mod unit_tests {
         assert!(matches!(err, Error::BackingFileOverlapsHeader(_, _, _)));
     }
 
+    #[test]
+    fn backing_file_size_without_offset() {
+        let err = read_header_with_patched_backing(0, 16).unwrap_err();
+        assert!(matches!(err, Error::BackingFileSizeWithoutOffset(16)));
+    }
+
     #[test]
     fn backing_file_fits_at_cluster_end() {
         let cluster_size = 1u64 << DEFAULT_CLUSTER_BITS;