[PATCH cairo 1/3] script-surface: Check for invalid ids (CID #1159557, 1159558)

When the bitmap's min is non-zero, _bitmap_next_id() may break out of
its loop early, before initializing the prev variable.  prev is then
dereferenced without a null ptr check.

Same issue is present in trace.c.

Coverity IDs: #1159557, #1159558

Signed-off-by: Bryce Harrington <[email protected]>
---
 src/cairo-script-surface.c | 3 ++-
 util/cairo-trace/trace.c   | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/cairo-script-surface.c b/src/cairo-script-surface.c
index e715cae..228d60e 100644
--- a/src/cairo-script-surface.c
+++ b/src/cairo-script-surface.c
@@ -267,7 +267,8 @@ _bitmap_next_id (struct _bitmap *b,
     if (unlikely (bb == NULL))
 	return _cairo_error (CAIRO_STATUS_NO_MEMORY);
 
-    *prev = bb;
+    if (prev != NULL)
+	*prev = bb;
     bb->next = b;
     bb->min = min;
     bb->count = 1;
diff --git a/util/cairo-trace/trace.c b/util/cairo-trace/trace.c
index 3c05613..269e876 100644
--- a/util/cairo-trace/trace.c
+++ b/util/cairo-trace/trace.c
@@ -301,7 +301,9 @@ _type_next_token (Type *t)
     }
 
     bb = malloc (sizeof (struct _bitmap));
-    *prev = bb;
+
+    if (prev != NULL)
+	*prev = bb;
     bb->next = b;
     bb->min = min;
     bb->count = 1;
-- 
2.7.4

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