Fix two Clang static analyzer issues

Overflowing memory allocations should crash or exit anyway.

src/common.c

@@ -1072,6 +1072,8 @@ poller_ensure_space (struct poller *self)
 		return;
 
 	self->alloc <<= 1;
+	hard_assert (self->alloc != 0);
+
 	self->revents = xreallocarray
 		(self->revents, sizeof *self->revents, self->alloc);
 	self->info = xreallocarray

src/zyklonb.c

@@ -773,7 +773,7 @@ plugin_process_message (const struct irc_message *msg,
 			return;
 
 		const char *command = msg->params.vector[0];
-		if (!strcasecmp (command, "register"))
+		if (!plugin->initialized && !strcasecmp (command, "register"))
 		{
 			// Register for relaying of IRC traffic
 			plugin->initialized = true;