[PATCH] fix coredump when length in ${name:off:length} is broken

["Jun. T" <takimoto-j@xxxxxxxxxxxxxxxxx>]

This is a patch for the bug #1 reported in workers/45843⁩:

> 2020/05/19 15:48, Aaron Esau <arinerron@xxxxxxxxxxxxxx> wrote:
> 
> ------ #1 :: null dereference in check_colon_subscript in subst.c ------
(snip)
> 
>  "${: :${{{\"{{i use arch btw}}"

We get coredump if the length part of ${name:offset:length} is seriously
broken; name and offset parts can be "normal". For example

"${foo:0:${\"}}"

gives coredump at line 3367 in subst.c:

3365:    check_offset = check_colon_subscript(check_offset2 + 1,
3366:                                         &check_offset2);
3367:    if (*check_offset2 && *check_offset2 != ':') {

When check_colon_subscript() returns NULL, check_offset2 is (not always
but usually) reset to NULL (line 1576).

With the patch below, "${foo:0:${\"}}" gives an error

unrecognized modifier `$'



diff --git a/Src/subst.c b/Src/subst.c
index 87a56c3c6..465fe970f 100644
--- a/Src/subst.c
+++ b/Src/subst.c
@@ -3362,13 +3362,15 @@ colonsubscript:
 		    return NULL;
 		}
 		if (*check_offset2) {
+		    char *nextp;
 		    check_offset = check_colon_subscript(check_offset2 + 1,
-							 &check_offset2);
-		    if (*check_offset2 && *check_offset2 != ':') {
-			zerr("invalid length: %s", check_offset);
-			return NULL;
-		    }
+							 &nextp);
 		    if (check_offset) {
+			check_offset2 = nextp;
+			if (*check_offset2 && *check_offset2 != ':') {
+			    zerr("invalid length: %s", check_offset);
+			    return NULL;
+			}
 			length = mathevali(check_offset);
 			length_set = 1;
 			if (errflag)
diff --git a/Test/D04parameter.ztst b/Test/D04parameter.ztst
index 05bb61bdc..b6b1f2e33 100644
--- a/Test/D04parameter.ztst
+++ b/Test/D04parameter.ztst
@@ -2664,10 +2664,9 @@ F:behavior, see http://austingroupbugs.net/view.php?id=888
 >3: pw
 >4: pw
 
- # Using a subshell because it segfaults.
- ("${: :${{{\"{{lorem ipsum dolor sit amet}}")
--f:regression test for workers/45843#1
-?(eval):1: bad substitution
+ : "${foo:0:${\"}}"
+1:broken length in ${name:offset:length} (workers/45843#1)
+?(eval):1: unrecognized modifier `$'
 
  $ZTST_testdir/../Src/zsh -fc $'$\\\n('
 1:regression test for workers/45843#2: escaped newline in command substitution start token