branches/zip: row_ext_cache_fill(): Add an assertion that some BLOB data

was actually fetched.  ext->len[i] == 0 is reserved to mean an uninitialized
BLOB pointer.

row/row0ext.c

@@ -43,6 +43,7 @@ row_ext_cache_fill(
 	ext->len[i] = btr_copy_externally_stored_field_prefix(
 		buf,
 		REC_MAX_INDEX_COL_LEN, zip_size, field, f_len);
+	ut_a(ext->len[i]);
 }
 
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