Fix a simple mistake with pattern space, and add a test for it

testsuite/sed/sed-append-hold-space-to-pattern-space

@@ -0,0 +1,13 @@
+busybox sed 'G'>output <<EOF
+a
+b
+c
+EOF
+cmp -s output - <<EOF
+a
+
+b
+
+c
+
+EOF