Add boundary checks for Ap4StsdAtom

Same problem as before, underflow of bytes_available results in some potential
attack

Source/C++/Core/Ap4StsdAtom.cpp

@@ -87,6 +87,7 @@ AP4_StsdAtom::AP4_StsdAtom(AP4_UI32         size,
                            AP4_AtomFactory& atom_factory) :
     AP4_ContainerAtom(AP4_ATOM_TYPE_STSD, size, false, version, flags)
 {
+    if (size < AP4_FULL_ATOM_HEADER_SIZE + 4) return;
     // read the number of entries
     AP4_UI32 entry_count;
     stream.ReadUI32(entry_count);