script-surface: Check for invalid ids (CID #1159557, 1159558)

If the bitmap's min is non-zero, _bitmap_next_id() could break out of
its loop early, before initializing the prev variable.  prev would then
be dereferenced without a null ptr check.  This condition should never
occur in practice, so add an assert() to assure it doesn't.

Same issue is present in trace.c.

Coverity IDs: #1159557, #1159558
Reviewed-By: Uli Schlachter <psychon@znc.in>
Signed-off-by: Bryce Harrington <bryce@bryceharrington.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/util/cairo-trace/trace.c b/util/cairo-trace/trace.c
index 3c056134e..87b2df46e 100644
--- a/util/cairo-trace/trace.c
+++ b/util/cairo-trace/trace.c
@@ -299,8 +299,10 @@ _type_next_token (Type *t)
 	prev = &b->next;
 	b = b->next;
     }
+    assert (prev != NULL);
 
     bb = malloc (sizeof (struct _bitmap));
+
     *prev = bb;
     bb->next = b;
     bb->min = min;