Unsanitized NUL in environment variables on Windows in syscall and os/exec

Due to unsanitized NUL values, attackers may be able to maliciously set environment variables on Windows.

In syscall.StartProcess and os/exec.Cmd, invalid environment variable values containing NUL values are not properly checked for. A malicious environment variable value can exploit this behavior to set a value for a different environment variable. For example, the environment variable string "A=B\x00C=D" sets the variables "A=B" and "C=D".