Fix UB in check_timeouts shift and CRC reflect

[Pivok7]

I made bindings of this library for the the Zig language and found out that it sometimes crashed. Zig detects unsigned out of bounds shifts in debug builds:

thread 36633 panic: shift exponent 32 is too large for 32-bit type 'unsigned int'
zig-pkg/N-V-__8AAJZTBQAYHEqBtyGRzh0esiAmJEjYMWoX8BNgTeov/include/enet.h:2972:22: 0x212e60a in enet_protocol_check_timeouts (/home/pivok/Dev/PiVoxel/zig-pkg/enet-2.7.0-FcQynOt5AAAreHzvVVZ9Em6Q3wJm6is-1vsG4NV8kjvA/src/enet.c)
                ((1u << (outgoingCommand->sendAttempts - 1)) >= peer->timeoutLimit &&

Two UB shift fixes:

1. enet_protocol_check_timeouts: sendAttempts is an unbounded enet_uint16 used as the exponent in 1u << (sendAttempts - 1). When sendAttempts reaches 33, the shift by 32 overflows unsigned int (UB). Under heavy load with a low-RTT peer, sendAttempts can grow past 32 before timeoutMinimum fires.
2. This one is just something i found out by looking for other UB shifts and it didn't acrually crash my program. reflect_crc: with bits=32, the expression 1 << (bits - 1 - bit) evaluates to 1 << 31 which overflows signed int (UB). Use 1u for a well-defined unsigned shift.