Fix use-after-free due to unexpected unref of context->default_style.
Segfault occurred when displaying two windows on different X11 displays.
valgrind said:
Invalid read of size 8
at 0x4A20962: _gtk_style_init_for_settings (gtkstyle.c:398)
by 0x49ED785: gtk_rc_get_style (gtkrc.c:2036)
by 0x4AC39C2: gtk_widget_reset_rc_style (gtkwidget.c:6601)
Address 0x70b8e80 is 560 bytes inside a block of size 1,024 free'd
at 0x48399AB: free (vg_replace_malloc.c:530)
by 0x51180D6: g_type_free_instance (in /usr/lib/libgobject-2.0.so.0.6000.6)
by 0x49E98CE: gtk_rc_reset_styles (gtkrc.c:1702)
by 0x49E9A0C: _gtk_rc_context_get_default_font_name (gtkrc.c:1740)
by 0x4A20961: _gtk_style_init_for_settings (gtkstyle.c:396)
by 0x49ED785: gtk_rc_get_style (gtkrc.c:2036)
by 0x4AC39C2: gtk_widget_reset_rc_style (gtkwidget.c:6601)
Block was alloc'd at
at 0x483877F: malloc (vg_replace_malloc.c:299)
by 0x51BD289: g_malloc (in /usr/lib/libglib-2.0.so.0.6000.6)
by 0x519F673: g_slice_alloc (in /usr/lib/libglib-2.0.so.0.6000.6)
by 0x51A62BA: g_slice_alloc0 (in /usr/lib/libglib-2.0.so.0.6000.6)
by 0x5119141: g_type_create_instance (in /usr/lib/libgobject-2.0.so.0.6000.6)
by 0x513563D: ??? (in /usr/lib/libgobject-2.0.so.0.6000.6)
by 0x5136A54: g_object_new_with_properties (in /usr/lib/libgobject-2.0.so.0.6000.6)
by 0x5136B51: g_object_new (in /usr/lib/libgobject-2.0.so.0.6000.6)
by 0x49ED774: gtk_rc_get_style (gtkrc.c:2035)
by 0x4AC39C2: gtk_widget_reset_rc_style (gtkwidget.c:6601)