uri: Fix compiler warnings with gcc

uri: Fix compiler warnings with gcc

These are actually not true.

gsturi.c: In function '_gst_uri_string_to_table.constprop':
gsturi.c:1316:27: error: 'pct_kv_sep' may be used uninitialized in this function [-Werror=maybe-uninitialized]
             for (next_sep = strcasestr (value, pct_kv_sep); next_sep;
                           ^
gsturi.c:1283:24: error: 'pct_part_sep' may be used uninitialized in this function [-Werror=maybe-uninitialized]
               next_sep = strcasestr (next_sep + 1, pct_part_sep)) {
                        ^