Fix MSVC warnings in deflate_quick_block_open

Fix MSVC warnings in deflate_quick_block_open

Add casts in order to fix the following warnings [1]:

C:\Users\Nathan\Source\zlib-ng\test\deflate_quick_block_open.c(62,69): warning C4244: '=': conversion from '__int64' to
 'uint32_t', possible loss of data [C:\Users\Nathan\Source\zlib-ng\deflate_quick_block_open.vcxproj]
C:\Users\Nathan\Source\zlib-ng\test\deflate_quick_block_open.c(73,1): warning C4244: 'initializing': conversion from '_
_int64' to 'uint32_t', possible loss of data [C:\Users\Nathan\Source\zlib-ng\deflate_quick_block_open.vcxproj]

[1] https://github.com/zlib-ng/zlib-ng/pull/880#issuecomment-802432700