Fix msg if not enough data for tile_size_minus_1.

If the read_is_valid(*data, tile_size_bytes, data_end) call in
get_tile_buffer() fails, it means there is not enough data for the
tile_size_minus_1 syntax element. The original error message
"Truncated packet or corrupt tile length" was apparently copied from
another read_is_valid() call and does not make sense here. Change the
error message to describe what this error means.

Change-Id: I4277f70100fe7eed0aea3b2bc46f944ac03892e0
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 0d94b57..71c8384 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2600,7 +2600,7 @@
   if (!is_last) {
     if (!read_is_valid(*data, tile_size_bytes, data_end))
       aom_internal_error(error_info, AOM_CODEC_CORRUPT_FRAME,
-                         "Truncated packet or corrupt tile length");
+                         "Not enough data to read tile size");
 
     size = mem_get_varsize(*data, tile_size_bytes) + AV1_MIN_TILE_SIZE_BYTES;
     *data += tile_size_bytes;