crt: Fix overflow detection in mingw-w64 fgetpos() emulation

Recent Windows versions have different behavior of 32-bit fgetpos and
returns -1 with errno set to EINVAL when position does not fit into 32 bits.

So check for EINVAL and convert it to EOVERFLOW.

This fixes t_fseeki64.c test when compiled against msvcrt20.dll and run on
Windows 10 against the system os version of msvcrt20.dll.