tidy: Fix a bunch of null dereference warnings in test_mpegtables.

The clang-tidy "null dereference" checker pointed out a bunch of
potential null dereferences in the test_mpegtables code.  The
dereferences are not obvious, as they occur inside of functions from
mpegdescriptors.h that have been inlined into the test code.  Adding
simple tests for descriptor validity, and exiting when they aren't
valid, eliminates all these warnings.

mythtv/libs/libmythtv/test/test_mpegtables/test_mpegtables.cpp

@@ -263,13 +263,20 @@ void TestMPEGTables::PrivateUPCCablecomEpisodetitleDescriptor_test (void)
 void TestMPEGTables::ItemList_test (void)
 {
     ShortEventDescriptor descriptor(&eit_data_0000[26]);
-    QVERIFY  (descriptor.IsValid());
+    if (!descriptor.IsValid()) {
+        QFAIL("The eit_data_0000 descriptor is invalid");
+        return;
+    }
     QCOMPARE (descriptor.DescriptorTag(), (unsigned int) DescriptorID::short_event);
     QCOMPARE (descriptor.size(), (unsigned int) 194);
     QCOMPARE (descriptor.LanguageString(), QString("ger"));
     QVERIFY  (descriptor.Text().startsWith(QString("Krimiserie. ")));
 
     ExtendedEventDescriptor descriptor2(&eit_data_0000[26+descriptor.size()]);
+    if (!descriptor2.IsValid()) {
+        QFAIL("The eit_data_0000 descriptor2 is invalid");
+        return;
+    }
     QCOMPARE (descriptor2.DescriptorTag(), (unsigned int) DescriptorID::extended_event);
     /* tests for items start here */
     QCOMPARE (descriptor2.LengthOfItems(), (uint) 139);
@@ -306,6 +313,10 @@ void TestMPEGTables::ParentalRatingDescriptor_test (void)
 void TestMPEGTables::ExtendedEventDescriptor_test (void)
 {
     ExtendedEventDescriptor desc(&eit_data_0000[16*13+12]);
+    if (!desc.IsValid()) {
+        QFAIL("The eit_data_0000 descriptor is invalid");
+        return;
+    }
     QCOMPARE (desc.LengthOfItems(), 139U);
     QMultiMap<QString,QString> items = desc.Items();
     QCOMPARE (items.count(), 5);