Clarify the peer pointer's validity using an assertion (CWE-476)

The prior switch statement dereferences the peer pointer in each valid
case, so if peer really was NULL then the program will have crashed
due to derefercing a null pointer.

Thus, this peer != NULL statement is misleading: it implies that peer
might legitimately be NULL, when infact, we expect peer to be valid at
this point.

So instead, we replace this checking with our expectation (using an
assertion).

Flagged by Coverity:

In enet_protocol_handle_incoming_commands(_ENetHost *, _ENetEvent *):
All paths that lead to this null pointer comparison already dereference
the pointer earlier (CWE-476)

include/enet.h

@@ -2641,7 +2641,8 @@ extern "C" {
                     goto commandError;
             }
 
-            if (peer != NULL && (command->header.command & ENET_PROTOCOL_COMMAND_FLAG_ACKNOWLEDGE) != 0) {
+            assert(peer);
+            if ((command->header.command & ENET_PROTOCOL_COMMAND_FLAG_ACKNOWLEDGE) != 0) {
                 enet_uint16 sentTime;
 
                 if (!(flags & ENET_PROTOCOL_HEADER_FLAG_SENT_TIME)) {