secur32: check for variable before freeing it

[Austin English]

Caused winetest64 to crash for me on Karmic. With this patch, runs
fine. See bug 20201.

-- 
-Austin
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diff --git a/dlls/secur32/schannel.c b/dlls/secur32/schannel.c
index f776206..dc003e9 100644
--- a/dlls/secur32/schannel.c
+++ b/dlls/secur32/schannel.c
@@ -1435,7 +1435,7 @@ void SECUR32_deinitSchannelSP(void)
         {
             struct schan_credentials *cred;
             cred = schan_free_handle(i, SCHAN_HANDLE_CRED);
-            pgnutls_certificate_free_credentials(cred->credentials);
+            if(cred) pgnutls_certificate_free_credentials(cred->credentials);
             HeapFree(GetProcessHeap(), 0, cred);
         }
     }