glapi/glx: Add overflow checks to the client-side indirect code

glapi/glx: Add overflow checks to the client-side indirect code

Coverity complains that the computed sizes can lead to negative lengths
passed to memcpy. If that happens we've been handed invalid arguments
anyway, so just bomb out.

The funky "0%s" is because the size string for the variable-length part
of the request is of the form "+ safe_pad() ...", and a unary + would
coerce the result to always be positive, defeating the overflow check.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>