hush: fix handling of heredocs starting with empty lines

function                                             old     new   delta
parse_stream                                        2748    2759     +11

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

shell/ash_test/ash-heredoc/heredoc_empty3.right

@@ -0,0 +1,2 @@
+
+Ok

shell/ash_test/ash-heredoc/heredoc_empty3.tests

@@ -0,0 +1,4 @@
+cat <<EOF
+
+Ok
+EOF

shell/hush.c

@@ -4226,6 +4226,14 @@ static char *fetch_till_str(o_string *as_string,
 	int prev = 0; /* not \ */
 	int ch;
 
+	/* Starting with "" is necessary for this case:
+	 * cat <<EOF
+	 *
+	 * xxx
+	 * EOF
+	 */
+	heredoc.data = xzalloc(1); /* start as "", not as NULL */
+
 	goto jump_in;
 
 	while (1) {

shell/hush_test/hush-heredoc/heredoc_empty3.right

@@ -0,0 +1,2 @@
+
+Ok

shell/hush_test/hush-heredoc/heredoc_empty3.tests

@@ -0,0 +1,4 @@
+cat <<EOF
+
+Ok
+EOF