grep: fix -o match with empty string (suggested by Colin Watson <cjwatson@ubuntu.com>)

function                                             old     new   delta
grep_file                                           1216    1251     +35

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

testsuite/grep.tests

@@ -98,5 +98,9 @@ testing "grep -o does not loop forever" \
 	'grep -o "[^/]*$"' \
 	"test\n" \
 	"" "/var/test\n"
+testing "grep -o does not loop forever on zero-length match" \
+	'grep -o "" | head -n1' \
+	"" \
+	"" "test\n"
 
 exit $FAILCOUNT