git-compat-util.h: implement checked size_t to uint32_t conversion

git-compat-util.h: implement checked size_t to uint32_t conversion

In a similar fashion as other checked cast functions in this header
(such as `cast_size_t_to_ulong()` and `cast_size_t_to_int()`), implement
a checked cast function for going from a size_t to a uint32_t value.

This function will be utilized in a future commit which needs to make
such a conversion.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>