sed: fix handling of s/// which has empty matches

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

editors/sed.c

@@ -741,7 +741,7 @@ static int do_subst_command(sed_cmd_t *sed_cmd, char **line_p)
 		 * The match_count check is so not to break
 		 * echo "hi" | busybox sed 's/^/!/g'
 		 */
-		if (!G.regmatch[0].rm_so && !G.regmatch[0].rm_eo && match_count) {
+		if (!G.regmatch[0].rm_so && !G.regmatch[0].rm_eo && match_count && *line) {
 			pipe_putc(*line++);
 			continue;
 		}

testsuite/sed.tests

@@ -48,6 +48,7 @@ testing "sed accepts multiple -e" "sed -e 'i\' -e '1' -e 'a\' -e '3'" \
 
 # substitutions
 testing "sed -n" "sed -n -e s/foo/bar/ -e s/bar/baz/" "" "" "foo\n"
+testing "sed with empty match" "sed 's/z*//g'" "string\n" "" "string\n"
 testing "sed s//p" "sed -e s/foo/bar/p -e s/bar/baz/p" "bar\nbaz\nbaz\n" \
 	"" "foo\n"
 testing "sed -n s//p" "sed -ne s/abc/def/p" "def\n" "" "abc\n"