The following fields contain duration values and should be checked that they contain only positive values:
GstPeriodNode start
GstPeriodNode duration
GstMetricsRangeNode startTime
GstMetricsRangeNode duration
GstMPDNode mediaPresentationDuration
GstMPDNode minimumUpdatePeriod
GstMPDNode minBufferTime
GstMPDNode timeShiftBufferDepth
GstMPDNode maxSegmentDuration
GstMPDNode maxSubsegmentDuration

(I'm not sure if GstMPDNode's suggestedPresentationDelay is allowed to have negative values.)

These fields have type gint64 and are set to -1 if the information is not found in the xml file. Ideally they should have the type GstClockTime and a default value of GST_CLOCK_TIME_NONE if not present. But that require changes in the API and careful changes in the code, because they currently represent values measured in milliseconds and GstClockTime holds values measured in nanoseconds.

At least we should check that their values are positive and reject xml files with negative values for those fields.