Auto merge of #58913 - Milack27:patch_buf_reader, r=joshtriplett
Auto merge of #58913 - Milack27:patch_buf_reader, r=joshtriplett

Add new test case for possible bug in BufReader

When reading a large chunk from a BufReader, if all the bytes from the buffer have been already consumed, the internal buffer is bypassed entirely. However, it is not invalidated, and it's possible to access its contents using the `seek_relative` method, because it tries to reuse the existing buffer.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust