Free memory before exiting in order to

help tools understand that we're not leaking it.

PR:		bin/171634
Submitted by:	Erik Cederstrand <erik@cederstrand.dk>
Approved by:	cperciva
MFC after:	3 days

bin/df/df.c

@@ -290,6 +290,7 @@ main(int argc, char *argv[])
 			prtstat(&mntbuf[i], &maxwidths);
 	if (cflag)
 		prtstat(&totalbuf, &maxwidths);
+	free(mntbuf);
 	return (rv);
 }