Add Valgrind checks to compressed BLOB access.

storage/innodb_plugin/btr/btr0cur.c

@@ -3871,6 +3871,8 @@ btr_store_big_rec_extern_fields(
 			field_ref += local_len;
 		}
 		extern_len = big_rec_vec->fields[i].len;
+		UNIV_MEM_ASSERT_RW(big_rec_vec->fields[i].data,
+				   extern_len);
 
 		ut_a(extern_len > 0);
 
@@ -4507,6 +4509,7 @@ btr_copy_blob_prefix(
 		mtr_commit(&mtr);
 
 		if (page_no == FIL_NULL || copy_len != part_len) {
+			UNIV_MEM_ASSERT_RW(buf, copied_len);
 			return(copied_len);
 		}
 
@@ -4690,6 +4693,7 @@ btr_copy_externally_stored_field_prefix_low(
 				      space_id, page_no, offset);
 		inflateEnd(&d_stream);
 		mem_heap_free(heap);
+		UNIV_MEM_ASSERT_RW(buf, d_stream.total_out);
 		return(d_stream.total_out);
 	} else {
 		return(btr_copy_blob_prefix(buf, len, space_id,