Don't read out-of-range ContentTypeId from configuration

officecfg/registry/schema/org/openoffice/Office/Writer.xcs specifies -1 as
default for /org.openoffice.Office/Writer/Navigator/RootType, which now triggers
the assert in o3tl::enumarray::operator[] during JunitTest_toolkit_unoapi_1.

Regression introduced with cb45d0d4 "convert
CONTENT_TYPE to scoped enum."  (A better fix might be to replace
ContentTypeId::UNKNOWN with -1; to be followed-up upon.)

Change-Id: Id12c775cee8936ef2fd411da25558acabb5fa0c9

sw/source/uibase/utlui/navicfg.cxx

@@ -77,7 +77,12 @@ SwNavigationConfig::SwNavigationConfig() :
                     {
                         sal_uInt32 nTmp;
                         if (pValues[nProp] >>= nTmp)
+                        {
+                            if (nTmp > sal_uInt32(ContentTypeId::LAST)) {
+                                nTmp = sal_uInt32(ContentTypeId::UNKNOWN);
+                            }
                             nRootType = static_cast<ContentTypeId>(nTmp);
+                        }
                         break;
                     }
                     case 1: pValues[nProp] >>= nSelectedPos;   break;