cpu/lpc2387: Add missing assert(buff != NULL)

cpu/lpc2387/mci/lpc2387-mci.c

@@ -380,6 +380,8 @@ static int send_cmd(unsigned int idx, unsigned long arg, unsigned int rt, unsign
 {
     unsigned int s, mc;
 
+    assert(buff != NULL);
+
     if (idx & 0x80) {               /* Send a CMD55 prior to the specified command if it is ACMD class */
         if (!send_cmd(CMD55, (unsigned long)CardRCA << 16, 1, buff) /* When CMD55 is faild, */
            || !(buff[0] & 0x00000020)) {