renice: Avoid segfault if -n flag is given without argument.

Fixes:		65ee0a8495
MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D39623

usr.bin/renice/renice.c

@@ -94,7 +94,7 @@ main(int argc, char *argv[])
 			}
 			if (strcmp(*argv, "-n") == 0) {
 				/* may occur only once, prior to priority */
-				if (haveprio || incr)
+				if (haveprio || incr || argc < 2)
 					usage();
 				incr = true;
 				(void)argc--, argv++;

usr.bin/renice/tests/renice_test.sh

@@ -158,6 +158,11 @@ renice_delim_body() {
 	kill $pid
 }
 
+atf_test_case renice_incr_noarg
+renice_incr_noarg_body() {
+	atf_check -o empty -e ignore -s exit:1 renice -n
+}
+
 atf_init_test_cases() {
 	atf_add_test_case renice_abs_pid
 	atf_add_test_case renice_rel_pid
@@ -166,4 +171,5 @@ atf_init_test_cases() {
 	atf_add_test_case renice_abs_user
 	atf_add_test_case renice_rel_user
 	atf_add_test_case renice_delim
+	atf_add_test_case renice_incr_noarg
 }