[PATCH] Fix POSIX compliance in pgwin32_unsetenv()

I noticed that pgwin32_unsetenv() in src/port/win32env.c lacks the input
validation that its sibling function pgwin32_setenv() has (lines 126-132).

Without these checks, the function will crash on NULL input via
strlen(NULL), and will accept empty strings or strings containing '=' in
violation of POSIX.1-2008.

The attached patch adds the same validation that pgwin32_setenv already
does, making the two functions consistent. This is purely defensive -
it only affects callers passing invalid arguments.

regards,

Bryan Green