Fixed Bug #66815 imagecrop(): insufficient fix for NULL defer CVE-2013-7327

This amends commit 8f4a537, which aimed to correct NULL dereference because of
missing check of gdImageCreateTrueColor() / gdImageCreate() return value.  That
commit checks for negative crop rectangle width and height, but
gdImageCreate*() can also return NULL when width * height overflows.  Hence
NULL deref is still possible, as gdImageSaveAlpha() and gdImagePaletteCopy()
is called before dst == NULL check.

This moves NULL check to happen right after gdImageCreate*().  It also removes
width and height check before gdImageCreate*(), as the same check is done by
image create functions (with an extra warning).

From thoger redhat com
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