add to testsuite and fix yet another sed corner case

testsuite/sed.tests

@@ -143,6 +143,9 @@ testing "sed subst+write" \
 	"sed -e 's/i/z/' -e 'woutputw' input -; echo -n X; cat outputw" \
 	"thzngy\nagaznXthzngy\nagazn" "thingy" "again"
 rm outputw
+testing "sed trailing NUL" \
+	"sed 's/i/z/' input -" \
+	"a\0b\0\nc" "a\0b\0" "c"
 
 # Test end-of-file matching behavior