Boundary checks in Ap4ContainerAtom

The lack of boundary checks in Ap4ContainerAtom leads to an underflow
in size which then leads to existing validation checks to fail and hence allow
large malloc.

Source/C++/Core/Ap4ContainerAtom.cpp

@@ -136,6 +136,7 @@ AP4_ContainerAtom::AP4_ContainerAtom(Type             type,
                                      AP4_AtomFactory& atom_factory) :
     AP4_Atom(type, size, force_64)
 {
+    if (size < GetHeaderSize()) return;
     ReadChildren(atom_factory, stream, size-GetHeaderSize());
 }
 
@@ -151,6 +152,7 @@ AP4_ContainerAtom::AP4_ContainerAtom(Type             type,
                                      AP4_AtomFactory& atom_factory) :
     AP4_Atom(type, size, force_64, version, flags)
 {
+    if (size < GetHeaderSize()) return;
     ReadChildren(atom_factory, stream, size-GetHeaderSize());
 }