fix string out-of-bounds assert on gnome627759-1.ods

i.e. on soffice --headless --convert-to pdf gnome627759-1.ods

Change-Id: I01289d5afcc79b7180cc12f8ee345ac760b6a637

svl/source/numbers/zformat.cxx

@@ -4359,6 +4359,7 @@ bool SvNumberformat::ImpGetNumberOutput(double fNumber,
                 }
                 break;
             case NF_SYMBOLTYPE_BLANK:
+                if (rInfo.sStrArray[j].getLength() >= 2)
                     /*k = */ InsertBlanks(sStr, k, rInfo.sStrArray[j][1] );
                 break;
             case NF_SYMBOLTYPE_STRING:
@@ -4492,6 +4493,7 @@ bool SvNumberformat::ImpNumberFillWithThousands( OUStringBuffer& sBuff,  // numb
             }
             break;
         case NF_SYMBOLTYPE_BLANK:
+            if (rInfo.sStrArray[j].getLength() >= 2)
                 /*k = */ InsertBlanks(sBuff, k, rInfo.sStrArray[j][1] );
             break;
         case NF_SYMBOLTYPE_THSEP: