This commit resolves undefined behaviour runtime errors related
to misaligned memory access. E.g.

> openair3/NAS/COMMON/IES/FGSMobileIdentity.c:193:3:
> runtime error: store to misaligned address 0x608000021e2b for type 'uint16_t',
> which requires 2 byte alignment 0x608000021e2b: note: pointer points here 00 f1 ...

* ENCODE_U24: safely encode 24-bit values into a buffer
  by using memcpy to copy 3 bytes in network byte order (big-endian).
* ENCODE_U16: safely encodes a 16-bit value into a buffer, handling
  misalignment by memcpy 2 bytes to buffer in network byte order (big-endian).