grep: fix a case when -o loops forever

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/grep.tests

@@ -90,4 +90,9 @@ testing "grep -E -o prints all matches" \
 	"00:19:3E:00:AA:5E\n00:1D:60:3D:3A:FB\n00:22:43:49:FB:AA\n" \
 	"" "00:19:3E:00:AA:5E 00:1D:60:3D:3A:FB 00:22:43:49:FB:AA\n"
 
+testing "grep -o does not loop forever" \
+	'grep -o "[^/]*$"' \
+	"test\n" \
+	"" "/var/test\n"
+
 exit $FAILCOUNT