Fixed disk spanning failures due to changes in #799.

At the end of a disk, we return any bytes read before proceeding to
the next disk. This allows us to return MZ_EXIST_ERROR when the next
disk is not found.

mz_strm_split.c

@@ -243,10 +243,11 @@ int32_t mz_stream_split_read(void *stream, void *buf, int32_t size) {
         if (read == 0) {
             if (split->current_disk < 0) /* No more disks to goto */
                 break;
+            if (size != bytes_left) /* Report read from previous disk before switching */
+                break;
             err = mz_stream_split_goto_disk(stream, split->current_disk + 1);
-            if (err == MZ_EXIST_ERROR) {
+            if (err == MZ_EXIST_ERROR)
                 split->current_disk = -1;
-            }
             if (err != MZ_OK)
                 return err;
         }