net_imap: Add assertion guarding potential NULL dereference.

There has been a single incident where there was a segfault
in this strncmp call, but not enough details at this point
to determine why this happened. Add some assertions for
the meantime to provide better visibility into this in the
future, should this happen again.

LBBS-14 #close

nets/net_imap/imap_client.c

@@ -823,13 +823,16 @@ static struct imap_client *__load_virtual_mailbox(struct imap_session *imap, con
 	size_t pathlen;
 
 	if (imap->client) {
+		const char *virtprefix = imap->client->virtprefix;
 		/* Reuse the same connection if it's the same account. */
-		if (!strncmp(imap->client->virtprefix, path, imap->client->virtprefixlen)) {
+		bbs_assert_exists(path);
+		bbs_assert_exists(virtprefix);
+		if (!strncmp(virtprefix, path, imap->client->virtprefixlen)) {
 			bbs_debug(5, "Reusing existing active connection for %s\n", path);
 			*exists = 1;
 			return imap->client;
 		}
-		/* An optimization here is if the remote server supports the UNAUTHENTICATE capability,
+		/* A potential optimization here is if the remote server supports the UNAUTHENTICATE capability,
 		 * we can reuse the connection instead of establishing a new one
 		 * (if it's the same server (hostname), but different user/account)
 		 * Unfortunately, no major providers support the UNAUTHENTICATE extension,