st-private: Fix memory leak

==13810== 11,360 bytes in 1 blocks are definitely lost in loss record 18,574 of 18,765
==13810==    at 0x4005447: calloc (vg_replace_malloc.c:467)
==13810==    by 0x5191882: standard_calloc (gmem.c:107)
==13810==    by 0x51920A7: g_malloc0 (gmem.c:196)
==13810==    by 0x4056201: blur_pixels (st-private.c:466)
==13810==    by 0x40573B4: _st_create_shadow_cairo_pattern (st-private.c:710)
==13810==    by 0x4070746: st_theme_node_paint (st-theme-node-drawing.c:856)
==13810==    by 0x3FEFFFFF: ???

https://bugzilla.gnome.org/show_bug.cgi?id=649497