Always return either a dynamically allocated string or NULL from

expand().  Never return the name parameter, which could be a the buf[]
buffer which is allocated on the stack by getdeadletter() and which
would then be used after getdeadletter() has returned.

Reported by:	Coverity
CID:		1199383
MFC after:	1 week

usr.bin/mail/fio.c

@@ -367,10 +367,10 @@ expand(char *name)
 		name = savestr(xname);
 	}
 	if (!strpbrk(name, "~{[*?$`'\"\\"))
-		return (name);
+		return (savestr(name));
 	if (pipe(pivec) < 0) {
 		warn("pipe");
-		return (name);
+		return (NULL);
 	}
 	(void)snprintf(cmdbuf, sizeof(cmdbuf), "echo %s", name);
 	if ((sh = value("SHELL")) == NULL)