Do not use constant syscall.EAGAIN

[debugger-zhang]

Currently code has several usages of syscall.EAGAIN. However, if you compile the project in MSYS2 MinGW (see here for detail), the value of syscall.EAGAIN will be 536870918 but libavcodec still returns 11. Therefore it will results in errors. It should be replaced with a hardcoded constant with value 11.