Use NULL rather than 0. (#778)

There was few cases of using literal 0 instead of NULL in the context of
pointers.  While this was a legitimate C code, using NULL rather than 0 makes
the code clearer.
diff --git a/PC/bdist_wininst/install.c b/PC/bdist_wininst/install.c
index 17cc30d..4f9ef6c 100644
--- a/PC/bdist_wininst/install.c
+++ b/PC/bdist_wininst/install.c
@@ -2179,7 +2179,7 @@
     // to leave the library loaded)
     if (0 == (shell32=LoadLibrary("shell32.dll")))
         return FALSE;
-    if (0 == (pfnIsUserAnAdmin=(PFNIsUserAnAdmin)GetProcAddress(shell32, "IsUserAnAdmin")))
+    if (NULL == (pfnIsUserAnAdmin=(PFNIsUserAnAdmin)GetProcAddress(shell32, "IsUserAnAdmin")))
         return FALSE;
     return (*pfnIsUserAnAdmin)();
 }