When a Reader encounters an error, its internal state may be corrupted.
So Reset needs to be extra thorough, to avoid cascading errors.
Based on PR 37 by Sovianum.
Use 64- or 32-bit loads instead of reading a byte at a time. The
original C source did something like this, in a very C-ish way. It
needed to be simplified to translate it to Go. The exact way this works
was suggested by the assembly code in github.com/golang/snappy.
Andy Balholm