fix 500+ new assert failures on export

soffice.bin: svl/source/config/languageoptions.cxx:167:

static SvtScriptType SvtLanguageOptions::FromI18NToSvtScriptType(sal_Int16):
Assertion `false && nI18NType && "Unknown i18n::ScriptType"' failed.

assert is new since...

commit a7b7c64a
Author: Noel Grandin <noel@peralex.com>
Date:   Wed Apr 15 09:36:39 2015 +0200

    convert SCRIPTTYPE_ constants to scoped enum

this fix seems sane.

Change-Id: Ic308bd34ac4ac76dd9e679f4387eb3ccea7a46c1

sc/source/filter/excel/xestyle.cxx

@@ -874,6 +874,15 @@ sal_Int16 XclExpFontHelper::GetFirstUsedScript( const XclExpRoot& rRoot, const S
         pCurrSet = pCurrSet->GetParent();
     }
 
+    if (nScript == 0)
+        nScript = nDefScript;
+
+    if (nScript == 0)
+    {
+        OSL_FAIL( "XclExpFontHelper::GetFirstUsedScript - unknown script type" );
+        nScript = ApiScriptType::LATIN;
+    }
+
     return nScript;
 }