HTTP/3: null-terminate empty header value.

Header value returned from the HTTP parser is expected to be null-terminated or
have a spare byte after the value bytes.  When an empty header value was passed
by client in a literal header representation, neither was true.  This could
result in segfault.  The fix is to assign a literal empty null-terminated
string in this case.

Thanks to Andrey Kolyshkin.

src/http/v3/ngx_http_v3_parse.c

@@ -746,6 +746,7 @@ ngx_http_v3_parse_header_l(ngx_connection_t *c,
 
         st->literal.length = st->pint.value;
         if (st->literal.length == 0) {
+            st->value.data = (u_char *) "";
             goto done;
         }