Check valid in binaryBlob::getIndex()

The binary blob shouldn't return an index if it ends up being invalid.
That could cause a whole lot of issues if musicclass ends up parsing the
resulting struct.

With all that said, though, musicclass doesn't check the -1 sentinel
value anyway, even though it should, but that'll be fixed later.

desktop_version/src/BinaryBlob.cpp

@@ -147,7 +147,7 @@ int binaryBlob::getIndex(const char* _name)
 {
 	for (size_t i = 0; i < SDL_arraysize(m_headers); i += 1)
 	{
-		if (strcmp(_name, m_headers[i].name) == 0)
+		if (strcmp(_name, m_headers[i].name) == 0 && m_headers[i].valid)
 		{
 			return i;
 		}