Fixed delta updates bug wherein updates fail on empty file.

mmap()ing empty files caused EINVAL to be returned and _hashOfFile
would return an incorrect result. This has been rectified.

SUBinaryDeltaCommon.m

@@ -78,6 +78,11 @@ static void _hashOfFile(unsigned char* hash, FTSENT *ent)
         }
 
         size_t fileSize = (size_t)ent->fts_statp->st_size;
+        if (fileSize == 0) {
+            _hashOfBuffer(hash, NULL, 0);
+            return;
+        }
+
         void *buffer = mmap(0, fileSize, PROT_READ, MAP_FILE | MAP_PRIVATE, fileDescriptor, 0);
         if (buffer == (void*)-1) {
             close(fileDescriptor);