Add boundary checks to Ap4IproAtom

Fuzzer caught another large malloc. This is caused by lack of boundary check
resulting in the next atom (children atom) to artificially large "size"
due to the underflow in bytes_available.

Source/C++/Core/Ap4IproAtom.cpp

@@ -62,6 +62,10 @@ AP4_IproAtom::AP4_IproAtom(AP4_UI32         size,
                            AP4_AtomFactory& atom_factory) :
     AP4_ContainerAtom(AP4_ATOM_TYPE_IPRO, size, false, version, flags)
 {
+    if (size < AP4_FULL_ATOM_HEADER_SIZE + 2) {
+        return;
+    }
+
     // read the number of entries
     AP4_UI16 entry_count;
     stream.ReadUI16(entry_count);