maint: avoid useless "if (foo) free(foo)" pattern

My Coccinelle semantic patch finds a few more, because it also fixes up
the equally pointless conditional

    if (foo) {
        free(foo);
        foo = NULL;
    }

Result (feel free to squash it into your patch):

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

translate-all.c

@@ -731,10 +731,8 @@ void tb_free(TranslationBlock *tb)
 
 static inline void invalidate_page_bitmap(PageDesc *p)
 {
-    if (p->code_bitmap) {
-        g_free(p->code_bitmap);
-        p->code_bitmap = NULL;
-    }
+    g_free(p->code_bitmap);
+    p->code_bitmap = NULL;
     p->code_write_count = 0;
 }