Fix undefined behaviour in number serialisation.

[samhocevar]

When optimising, the compiler is allowed to reorder the read calls
in the following code:

return  (uint16_t)read_uint8_t(info) |
       ((uint16_t)read_uint8_t(info) << 8);

Since read_uint8_t() has side effects, this could read a totally
wrong value and has to be rewritten as such:

uint16_t value = (uint16_t)read_uint8_t(info);
value |= (uint16_t)read_uint8_t(info) << 8;
return value;

This bug has been observed with Visual Studio 2019.