Revert "Fix `_File::get_buffer` length always set to p_length"

This reverts commit 9a7e537aac0fd251cc500d39e41224c7bf2fdf48.

Fixes #47971.

core/bind/core_bind.cpp

@@ -2117,7 +2117,7 @@ PoolVector<uint8_t> _File::get_buffer(int p_length) const {
 	w.release();
 
 	if (len < p_length)
-		data.resize(len);
+		data.resize(p_length);
 
 	return data;
 }