mktemp: skip later TMPDIR check if -p is specified

We already did the necessary $TMPDIR fallback, if it's going to be used.
Skip the later check so that we don't accidentally override our -p
argument.

Fixes:		ac6f924e1c ("mktemp: add -p/--tmpdir argument")
Sponsored by:	Klara, Inc.

usr.bin/mktemp/mktemp.c

@@ -85,6 +85,12 @@ main(int argc, char **argv)
 			tmpdir = optarg;
 			if (tmpdir == NULL || *tmpdir == '\0')
 				tmpdir = getenv("TMPDIR");
+
+			/*
+			 * We've already done the necessary environment
+			 * fallback, skip the later one.
+			 */
+			prefer_tmpdir = false;
 			break;
 
 		case 'q':

usr.bin/mktemp/tests/mktemp_test.sh

@@ -69,7 +69,7 @@ tmpdir_pflag_body()
 	# Finally, combined -p -t
 	atf_check -o match:"^$pflag/foo\..+$" \
 	    env -u TMPDIR mktemp -p "$pflag" -t foo
-	atf_check -o match:"^$tmpdir/foo\..+$" \
+	atf_check -o match:"^$pflag/foo\..+$" \
 	    env TMPDIR="$tmpdir" mktemp -p "$pflag" -t foo
 }