st/mesa: copy num_immediates field when copying the immediates array

st/mesa: copy num_immediates field when copying the immediates array

Two assignments to num_immediates were missing in
get_pixel_transfer_visitor() and get_bitmap_visitor().
The uninitialized value led to valgrind errors and crashes in some
cases.

Added new assertions to catch future problems in this area.  Also
changed num_immediates to unsigned to avoid signed/unsigned
comparison warnings.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>