Revert "diff-delta.c: allow delta with empty blob."

This reverts 962537a commit to
play safe.

delta.h

@@ -9,8 +9,8 @@ extern void *patch_delta(void *src_buf, unsigned long src_size,
 			 void *delta_buf, unsigned long delta_size,
 			 unsigned long *dst_size);
 
-/* the smallest possible delta size is 2 bytes (empty to empty) */
-#define DELTA_SIZE_MIN	2
+/* the smallest possible delta size is 4 bytes */
+#define DELTA_SIZE_MIN	4
 
 /*
  * This must be called twice on the delta data buffer, first to get the

diff-delta.c

@@ -213,7 +213,7 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 	bdrecord_t *brec;
 	bdfile_t bdf;
 
-	if (delta_prepare(from_buf, from_size, &bdf))
+	if (!from_size || !to_size || delta_prepare(from_buf, from_size, &bdf))
 		return NULL;
 
 	outpos = 0;