render: Fix out of boundary heap access

ProcRenderCreateRadialGradient and ProcRenderCreateConicalGradient must
be protected against an integer overflow during length check. This is
already included in ProcRenderCreateLinearGradient since the fix for
CVE-2008-2362.

This can only be successfully exploited on a 32 bit system for an
out of boundary read later on. Validated by using ASAN.

Reviewed-by: Adam Jackson <ajax@redhat.com>