HTTP/3: fixed potential type overflow in string literal parser.

This might happen for Huffman encoded string literals as the result
of length expansion.  Notably, the maximum length of string literals
is already limited with the "large_client_header_buffers" directive,
so this was only possible with nonsensically large configured limits.

src/http/v3/ngx_http_v3_parse.c

@@ -623,6 +623,12 @@ ngx_http_v3_parse_literal(ngx_connection_t *c, ngx_http_v3_parse_literal_t *st,
             }
 
             if (st->huffman) {
+                if (n > NGX_MAX_INT_T_VALUE / 8) {
+                    ngx_log_error(NGX_LOG_INFO, c->log, 0,
+                                  "client sent too large field line");
+                    return NGX_HTTP_V3_ERR_EXCESSIVE_LOAD;
+                }
+
                 n = n * 8 / 5;
                 st->huffstate = 0;
             }