passing a NULL pointer to fileno is not allowed

See crash reports at
http://crashreport.libreoffice.org/stats/signature/do_msvcr_magic+0x7
and documentation at
https://msdn.microsoft.com/en-us/library/zs6wbdhx.aspx

Change-Id: Ia9166d3b9fa10b87585821504e39cdfecbd22eda
Reviewed-on: https://gerrit.libreoffice.org/26317Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

jvmfwk/plugins/sunmajor/pluginlib/sunjavaplugin.cxx

@@ -548,6 +548,9 @@ static void do_msvcr_magic(rtl_uString *jvm_dll)
 
     FILE *f = _wfopen(reinterpret_cast<LPCWSTR>(Module->buffer), L"rb");
 
+    if (!f)
+        return;
+
     if (fstat(fileno(f), &st) == -1)
     {
         fclose(f);