[Core] Fix File.get_buffer returning wrong length

File.get_buffer always returned as many bytes as requested (even when
EOF was reached), this resulted in random bytes being returned when
overflowing.

core/bind/core_bind.cpp

@@ -2177,7 +2177,7 @@ PoolVector<uint8_t> _File::get_buffer(int64_t p_length) const {
 	w.release();
 
 	if (len < p_length) {
-		data.resize(p_length);
+		data.resize(len);
 	}
 
 	return data;