Plug a mem leak in g-icon test

Plug a mem leak in g-icon test

==2428== 256 bytes in 1 blocks are definitely lost in loss record 591 of 604
==2428==    at 0x4005CD2: realloc (vg_replace_malloc.c:476)
==2428==    by 0x40571A5: g_realloc (gmem.c:181)
==2428==    by 0x4075287: g_string_maybe_expand (gstring.c:395)
==2428==    by 0x40760D8: g_string_insert_c (gstring.c:1049)
==2428==    by 0x4074D41: g_string_append_c_inline (gstring.h:153)
==2428==    by 0x4075B3C: g_string_append_uri_escaped (gstring.c:822)
==2428==    by 0x41A46AC: g_icon_to_string_tokenized (gicon.c:164)
==2428==    by 0x41A498F: g_icon_to_string (gicon.c:252)
==2428==    by 0x8049E1A: test_g_icon_serialize (g-icon.c:222)

Bug #628331.