Fix to make plugins work on windows (last system error code for

the thread wasn't getting reset). Somebody with more windows
saavy might want to check that this doesn't break other stuff.

source/blender/blenlib/intern/dynlib.c

@@ -67,8 +67,15 @@ void *PIL_dynlib_find_symbol(PILdynlib* lib, char *symname) {
 }
 
 char *PIL_dynlib_get_error_as_string(PILdynlib* lib) {
-	int err= GetLastError();
+	int err;
 
+	/* if lib is NULL reset the last error code */
+	if (!lib) {
+		SetLastError(ERROR_SUCCESS);
+		return NULL;
+	}
+
+	err= GetLastError();
 	if (err) {
 		static char buf[1024];