Fix multiple instances of undefined behaviour and crashes when using ubsan

[GasInfinity]

While compiling with zig cc I encountered crashes due to undefined behaviour (It enables ubsan by default in debug/safe builds). This PR aims to fix some code which causes ub

Some of the fixes include:

• Avoid passing NULL to src or dest in memcpy()
• Avoid incrementing/decrementing a NULL pointer
• Avoid incrementing a pointer by 0
• Avoid doing signed bit shifting by casting to unsigned first