CLEANUP: Apply ist.cocci

Make use of the new rules to use `istnext()`.
diff --git a/src/cache.c b/src/cache.c
index feab63f..ba2b63c 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -2180,49 +2180,49 @@
 
 	switch (*encoding.ptr) {
 	case 'a':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "aes128gcm", VARY_ENCODING_AES128GCM);
 		break;
 	case 'b':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "br", VARY_ENCODING_BR);
 		break;
 	case 'c':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "compress", VARY_ENCODING_COMPRESS);
 		break;
 	case 'd':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "deflate", VARY_ENCODING_DEFLATE);
 		break;
 	case 'e':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "exi", VARY_ENCODING_EXI);
 		break;
 	case 'g':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "gzip", VARY_ENCODING_GZIP);
 		break;
 	case 'i':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "identity", VARY_ENCODING_IDENTITY);
 		break;
 	case 'p':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "pack200-gzip", VARY_ENCODING_PACK200_GZIP);
 		break;
 	case 'x':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "x-gzip", VARY_ENCODING_GZIP);
 		if (!*encoding_value)
 			*encoding_value = CHECK_ENCODING(encoding, "x-compress", VARY_ENCODING_COMPRESS);
 		break;
 	case 'z':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = CHECK_ENCODING(encoding, "zstd", VARY_ENCODING_ZSTD);
 		break;
 	case '*':
-		encoding = istadv(encoding, 1);
+		encoding = istnext(encoding);
 		*encoding_value = VARY_ENCODING_STAR;
 		break;
 	default:
@@ -2238,7 +2238,7 @@
 				return -1;
 
 			if (has_null_weight) {
-				encoding = istadv(encoding, 1);
+				encoding = istnext(encoding);
 
 				encoding = http_trim_leading_spht(encoding);
 
diff --git a/src/http_htx.c b/src/http_htx.c
index bfdcaef..8028cfc 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -146,8 +146,7 @@
 			goto next_blk;
 		/* Skip comma */
 		if (*(v.ptr) == ',') {
-			v.ptr++;
-			v.len--;
+			v = istnext(v);
 		}
 
 		goto return_hdr;
@@ -216,8 +215,7 @@
 		ctx->lws_before = 0;
 		ctx->lws_after = 0;
 		while (v.len && HTTP_IS_LWS(*v.ptr)) {
-			v.ptr++;
-			v.len--;
+			v = istnext(v);
 			ctx->lws_before++;
 		}
 		if (!(flags & HTTP_FIND_FL_FULL))
@@ -457,16 +455,14 @@
 	uri = htx_sl_req_uri(sl);
 	q = uri;
 	while (q.len > 0 && *(q.ptr) != '?') {
-		q.ptr++;
-		q.len--;
+		q = istnext(q);
 	}
 
 	/* skip the question mark or indicate that we must insert it
 	 * (but only if the format string is not empty then).
 	 */
 	if (q.len) {
-		q.ptr++;
-		q.len--;
+		q = istnext(q);
 	}
 	else if (query.len > 1)
 		offset = 0;
diff --git a/src/mqtt.c b/src/mqtt.c
index 6624201..ebdb57d 100644
--- a/src/mqtt.c
+++ b/src/mqtt.c
@@ -183,7 +183,7 @@
 		return IST_NULL;
 	if (i)
 		*i = (uint8_t)*istptr(parser);
-	parser = istadv(parser, 1);
+	parser = istnext(parser);
 	return parser;
 }