Fix double-free in gdImageWebPtr()

The issue is that gdImageWebpCtx() (which is called by gdImageWebpPtr() and
the other WebP output functions to do the real work) does not return whether
it succeeded or failed, so this is not checked in gdImageWebpPtr() and the
function wrongly assumes everything is okay, which is not, in this case,
because there is a size limitation for WebP, namely that the width and
height must by less than 16383.

We can't change the signature of gdImageWebpCtx() for API compatibility
reasons, so we introduce the static helper _gdImageWebpCtx() which returns
success respective failure, so gdImageWebpPtr() and gdImageWebpPtrEx() can
check the return value. We leave it solely to libwebp for now to report
warnings regarding the failing write.

This issue had been reported by Ibrahim El-Sayed to security@libgd.org.

CVE-2016-6912