Do not error out when calling xfree(NULL)

Calling free() with NULL as parameter is perfectly safe, no need to
error out here.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/mem.c

@@ -121,7 +121,6 @@ char *xstrdup(const char *str)
 
 void xfree(void *p)
 {
-	EXIT_IF(p == NULL, _("xfree: null pointer"));
 	free(p);
 }