fix memory leak originating in purple_prefs_connect_callbackrelease-2.x.y

Testing Done:
This leak happened just by starting and closing Pidgin. So I started and closed Pidgin and the leak didn't happen anymore.

Example valgrind output of the leak:
```
==425973== 21 bytes in 1 blocks are definitely lost in loss record 3,984 of 17,678
==425973==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==425973==    by 0x54CF738: g_malloc (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7200.4)
==425973==    by 0x54E4583: g_strdup (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7200.4)
==425973==    by 0x5796FBE: purple_prefs_connect_callback (prefs.c:1425)
==425973==    by 0x17DF74: setup_menubar (gtkconv.c:3573)
==425973==    by 0x18F25F: pidgin_conv_window_new (gtkconv.c:9477)
==425973==    by 0x18BAAF: pidgin_conversations_init (gtkconv.c:8193)
==425973==    by 0x1BE505: pidgin_ui_init (gtkmain.c:328)
==425973==    by 0x57607F9: purple_core_init (core.c:188)
==425973==    by 0x1BF5C3: main (gtkmain.c:826)
```

Reviewed at https://reviews.imfreedom.org/r/2226/

1 files changed, 4 insertions, 1 deletions

diff --git a/libpurple/prefs.c b/libpurple/prefs.c
index d3cb30d216..56d46156d5 100644
--- a/libpurple/prefs.c
+++ b/libpurple/prefs.c
@@ -759,6 +759,7 @@ remove_pref(struct purple_pref *pref)
 	struct purple_pref *child = NULL, *next = NULL;
 	char *name;
 	GSList *l;
+	PurplePrefCallbackData *cb;
 
 	if(!pref)
 		return;
@@ -793,7 +794,9 @@ remove_pref(struct purple_pref *pref)
 
 	while((l = pref->callbacks) != NULL) {
 		pref->callbacks = pref->callbacks->next;
-		g_free(l->data);
+		cb = l->data;
+		g_free(cb->name);
+		g_free(cb);
 		g_slist_free_1(l);
 	}
 	g_free(pref->name);