Fix a leak when the contents of the theme aren't valid XML.

==2501== 7,153 bytes in 1 blocks are definitely lost in loss record 1,756 of 1,861
==2501== at 0x04c278ae: malloc (vg_replace_malloc.c:207)
==2501== by 0x09b122ab: g_file_get_contents (gfileutils.c:613)
==2501== by 0x112b7ba6: gf_theme_new_from_file (gf_theme.c:83)
==2501== by 0x112b7df9: gf_theme_probe (gf_theme.c:334)
==2501== by 0x112b8001: gf_themes_probe (gf_theme.c:372)
==2501== by 0x112beae6: gf_load (guifications.c:58)
==2501== by 0x07d1890b: purple_plugin_load (plugin.c:616)
==2501== by 0x07d193a0: purple_plugins_load_saved (plugin.c:1314)
==2501== by 0x00483e86: main (gtkmain.c:807)