libsigrok-internal.h: add length checking endianess helpers

libsigrok-internal.h: add length checking endianess helpers

Introduce the read_u8_inc_len() and read_u16le_inc_len() routines which
read the respective numbers from binary input, advance the current read
position, and also track the remaining length of input data. Call sites
need not duplicate the data field width they advanced over.

The input stream is considered exhausted when there is insufficient data
to read another field. All subsequent reads will also terminate. Calling
applications need to check before reading when they expect specific sets
of adjacent fields to be available. It is assumed that most call sites
already do this (once for the whole input stream, not individually for
each conversion call of a field).