Fix two thinkos related to strong random keys.

pg_backend_random() is used for MD5 salt generation, but it can fail, and
no checks were done on its status code.

Fix memory leak, if generating a random number for a cancel key failed.

Both issues were spotted by Coverity. Fix by Michael Paquier.

src/backend/postmaster/postmaster.c

@@ -3901,6 +3901,7 @@ BackendStartup(Port *port)
 	 */
 	if (!RandomCancelKey(&MyCancelKey))
 	{
+		free(bn);
 		ereport(LOG,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("could not acquire random number")));