bswap: convert to unsigned before shifting in get_be32

bswap: convert to unsigned before shifting in get_be32

The pointer p is dereferenced and we get an unsigned char.  Before
shifting it's automatically promoted to int.  Left-shifting a signed
32-bit value bigger than 127 by 24 places is undefined.  Explicitly
convert to a 32-bit unsigned type to avoid undefined behaviour if
the highest bit is set.

Found with Clang's UBSan.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>