sed: fix breakage added by zero length match code

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

testsuite/sed.tests

@@ -302,6 +302,10 @@ testing "sed zero chars match/replace advances correctly 2" \
 	"sed 's [^ .]* x g'" \
 	"x x.x\n" "" " a.b\n"
 
+testing "sed zero chars match/replace logic must not falsely trigger here" \
+	"sed 's/a/A/g'" \
+	"_AAA1AA\n" "" "_aaa1aa\n"
+
 # testing "description" "commands" "result" "infile" "stdin"
 
 exit $FAILCOUNT