sed: Fix handling of an empty pattern space

Add a regression test.

PR:		271791
Obtained from:	OpenBSD (1.38 millert)
MFC after:	2 weeks

(cherry picked from commit 5982237)

Author: mohd-akram

usr.bin/sed/process.c

@@ -439,14 +439,12 @@ substitute(struct s_command *cp)
 		 * and at the end of the line, terminate.
 		 */
 		if (match[0].rm_so == match[0].rm_eo) {
-			if (*s == '\0' || *s == '\n')
-				slen = -1;
-			else
-				slen--;
-			if (*s != '\0') {
+			if (slen > 0) {
 			 	cspace(&SS, s++, 1, APPEND);
+				slen--;
 				le++;
-			}
+			} else
+				slen = -1;
 			lastempty = 1;
 		} else
 			lastempty = 0;

usr.bin/sed/tests/sed2_test.sh

@@ -159,6 +159,19 @@ minus_e_body()
 	atf_check -o 'inline:--\nab\n' sed    $'1 i\\\n--' a
 }
 
+atf_test_case command_D
+command_D_head()
+{
+	atf_set "descr" "Test handling of an empty pattern space"
+}
+command_D_body()
+{
+	printf "hello\n\nworld\n" > a
+
+	atf_check -o file:a sed -e 's/^//;P;D' a
+	atf_check -o file:a sed -e 's/^//;$!N;P;D' a
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case inplace_command_q
@@ -169,4 +182,5 @@ atf_init_test_cases()
 	atf_add_test_case hex_subst
 	atf_add_test_case bracket_y
 	atf_add_test_case minus_e
+	atf_add_test_case command_D
 }