Correct pUniFreeUconvObject type and add null check

Fix function pointer type for pUniFreeUconvObject and add null check before calling it.

(cherry picked from commit 49cfd5f)

Author: josch1710

src/physfs_platform_os2.c

@@ -33,7 +33,7 @@
 static HMODULE uconvdll = 0;
 static UconvObject uconv = 0;
 static int (_System *pUniCreateUconvObject)(UniChar *, UconvObject *) = NULL;
-static int (_System *pUniFreeUconvObject)(UconvObject *) = NULL;
+static int (_System *pUniFreeUconvObject)(UconvObject) = NULL;
 static int (_System *pUniUconvToUcs)(UconvObject,void **,size_t *, UniChar**, size_t *, size_t *) = NULL;
 static int (_System *pUniUconvFromUcs)(UconvObject,UniChar **,size_t *,void **,size_t *,size_t *) = NULL;
 
@@ -303,7 +303,8 @@ void __PHYSFS_platformDeinit(void)
 {
     if (uconvdll)
     {
-        pUniFreeUconvObject(uconv);
+        if (uconv)
+            pUniFreeUconvObject(uconv);
         uconv = 0;
         DosFreeModule(uconvdll);
         uconvdll = 0;