Fix setenv() with value==NULL on Windows

Also comment about putenv() taking a *copy* of the string. That is
Windows-specific behaviour and not POSIX-compliant, so we don't want
anyone thinking it's reasonable to do it elsewhere:

Tested thus, under wine and Windows 7:

int main(void)
{
	char foo[]="FOO=bar";
	char *bar;

	putenv(foo);
	printf("FOO is: %s\n", getenv("FOO"));
	foo[5] = 'f';
	printf("FOO is: %s\n", getenv("FOO"));
	putenv("FOO=");
	printf("FOO is: %s\n", getenv("FOO"));
}

With the following results:

FOO is: bar
FOO is: bar
FOO is: (null)

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

compat.c

@@ -166,7 +166,14 @@ char *openconnect__strcasestr(const char *haystack, const char *needle)
 #ifndef HAVE_SETENV
 int openconnect__setenv(const char *name, const char *value, int overwrite)
 {
-	char *buf = alloca(strlen(name) + strlen(value) + 2);
+	char *buf;
+
+	if (!value) {
+		openconnect__unsetenv(name);
+		return 0;
+	}
+	/* Windows putenv() takes a copy of the string */
+	buf = alloca(strlen(name) + strlen(value) + 2);
 
 	sprintf(buf, "%s=%s", name, value);
 	putenv(buf);