Debugging uint16_t shows incorrect value (-1) when used from a method that comes through an "extern C"

If I step into the u8s_to_16 function (inside the attached example) which is included through `extern C` the values suddenly switch to -1 when the shift `<< 8` happens, if using the same code in a .cpp file (see main.cpp) then everything is shown correctly.
If I use the gdb from the console and step through the program, the values are displayed correctly.