SDL_LoadObject (windows, os/2): prevent crash if dll name is NULL.

src/loadso/windows/SDL_sysloadso.c

@@ -32,7 +32,15 @@
 void *
 SDL_LoadObject(const char *sofile)
 {
-    LPTSTR tstr = WIN_UTF8ToString(sofile);
+    LPTSTR tstr;
+
+    if (!sofile) {
+        SDL_SetError("NULL sofile");
+        return NULL;
+    }
+
+    tstr = WIN_UTF8ToString(sofile);
+
 #ifdef __WINRT__
     /* WinRT only publically supports LoadPackagedLibrary() for loading .dll
        files.  LoadLibrary() is a private API, and not available for apps