MDEV-18659: Revert a non-functional change

fts_fetch_index_words(): Restore the initialization len=0.
The test innodb_fts.create in 10.2 would end up in an infinite loop
if this assignment is removed, because a following iteration of the
while() loop would assign zip->zp->avail_in=len with the original value
instead of the 0 that was reset in the previous iteration.

storage/innobase/fts/fts0opt.cc

@@ -717,15 +717,15 @@ fts_fetch_index_words(
 				zip->zp->next_in = static_cast<byte*>(data);
 				zip->zp->avail_in = len;
 				ut_a(len <= FTS_MAX_WORD_LEN);
+				len = 0;
 			}
-			break;
+			continue;
 
 		case Z_STREAM_END:
 		case Z_BUF_ERROR:
 		case Z_STREAM_ERROR:
 		default:
 			ut_error;
-			break;
 		}
 	}
 

storage/xtradb/fts/fts0opt.cc

@@ -717,15 +717,15 @@ fts_fetch_index_words(
 				zip->zp->next_in = static_cast<byte*>(data);
 				zip->zp->avail_in = len;
 				ut_a(len <= FTS_MAX_WORD_LEN);
+				len = 0;
 			}
-			break;
+			continue;
 
 		case Z_STREAM_END:
 		case Z_BUF_ERROR:
 		case Z_STREAM_ERROR:
 		default:
 			ut_error;
-			break;
 		}
 	}