Avoid overflow in 'last' variable of FindGLXFunction(...)

Avoid overflow in 'last' variable of FindGLXFunction(...)

This 'last' variable used in FindGLXFunction(...) may become negative,
but has been defined as unsigned int resulting in an overflow,
finally resulting in a segfault when accessing _glXDispatchTableStrings[...].
Fixed this by definining it as signed int. 'first' variable also needs to be
defined as signed int. Otherwise condition for while loop fails due to C
implicitly converting signed to unsigned values before comparison.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Stefan Dirsch <sndirsch@suse.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>