sh: Fix use after free when resetting an in-use alias.

The special case of modifying an existing alias does not work correctly if
the alias is currently in use. Instead, handle this case by unaliasing the
old alias (if any) and then creating a new alias.

bin/sh/alias.c

@@ -63,17 +63,8 @@ setalias(const char *name, const char *val)
 {
 	struct alias *ap, **app;
 
+	unalias(name);
 	app = hashalias(name);
-	for (ap = *app; ap; ap = ap->next) {
-		if (equal(name, ap->name)) {
-			INTOFF;
-			ckfree(ap->val);
-			ap->val	= savestr(val);
-			INTON;
-			return;
-		}
-	}
-	/* not found */
 	INTOFF;
 	ap = ckmalloc(sizeof (struct alias));
 	ap->name = savestr(name);

bin/sh/tests/parser/Makefile

@@ -24,6 +24,7 @@ ${PACKAGE}FILES+=	alias14.0
 ${PACKAGE}FILES+=	alias15.0 alias15.0.stdout
 ${PACKAGE}FILES+=	alias16.0
 ${PACKAGE}FILES+=	alias17.0
+${PACKAGE}FILES+=	alias18.0
 ${PACKAGE}FILES+=	and-pipe-not.0
 ${PACKAGE}FILES+=	case1.0
 ${PACKAGE}FILES+=	case2.0

bin/sh/tests/parser/alias18.0

@@ -0,0 +1,8 @@
+# $FreeBSD$
+
+v=1
+alias a='alias a=v=2
+v=3
+a'
+eval a
+[ "$v" = 2 ]