misc: Use safe_cast when assumptions are made about return value

This patch changes two dynamic_cast to safe_cast as we assume the
return value is not NULL (without checking).

src/cpu/testers/memtest/memtest.cc

@@ -170,7 +170,7 @@ MemTest::completeRequest(PacketPtr pkt)
             pkt->isError() ? "error" : "success");
 
     MemTestSenderState *state =
-        dynamic_cast<MemTestSenderState *>(pkt->senderState);
+        safe_cast<MemTestSenderState *>(pkt->senderState);
 
     uint8_t *data = state->data;
     uint8_t *pkt_data = pkt->getPtr<uint8_t>();

src/mem/packet.cc

@@ -189,7 +189,7 @@ Packet::checkFunctional(Printable *obj, Addr addr, bool is_secure, int size,
 
     // check print first since it doesn't require data
     if (isPrint()) {
-        dynamic_cast<PrintReqState*>(senderState)->printObj(obj);
+        safe_cast<PrintReqState*>(senderState)->printObj(obj);
         return false;
     }