tdf#107135 fix too tight constraint for FINDB.

Change-Id: I72e7214f14840146305fcb7daec7db89fcf112c9
Reviewed-on: https://gerrit.libreoffice.org/41959Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>

sc/source/core/tool/interpr1.cxx

@@ -8783,7 +8783,7 @@ void ScInterpreter::ScFindB()
         int nLen       = getLengthB( aStr );
         OUString asStr = GetString().getString();
         int nsLen      = getLengthB( asStr );
-        if ( nStart < 1 || nStart > nLen - nsLen )
+        if ( nStart < 1 || nStart > nLen - nsLen + 1 )
             PushIllegalArgument();
         else
         {