Fix potential null pointer dereferences in EITHelper::AddEIT.

Ensure that a ContentDescriptor is valid before trying to extract data
from it.  If its invalid, use "unknown" for the category type instead
of dereferencing a nullptr.

mythtv/libs/libmythtv/eithelper.cpp

@@ -701,11 +701,16 @@ void EITHelper::AddEIT(const PremiereContentInformationTable *cit)
                 category = QObject::tr("Sports");
             }
         }
-        else
+        else if (content.IsValid())
         {
             category_type = content.GetMythCategory(0);
             category      = content.GetDescription(0);
         }
+        else
+        {
+            category_type = ProgramInfo::kCategoryNone;
+            category      = "Unknown";
+        }
     }
 
     uint tableid   = cit->TableID();