Fix getpwd() for non-glibc systems.

In commit e048cdd I changed getpwd() to no longer call savestr(),
choosing to keep the path in the already available buffer, but neglected
to make the buffer available after the function returned.

Reported by Martijn Dekker.

Test case:

  pwd -P

src/cd.c

@@ -312,7 +312,7 @@ getpwd(int flags)
 	if (dir)
 		return dir;
 #else
-	char buf[PATH_MAX];
+	static char buf[PATH_MAX];
 
 	if (getcwd(buf, sizeof(buf)))
 		return buf;