open_istream: remove unneeded check for null pointer

open_istream: remove unneeded check for null pointer

'st' is allocated via xmalloc a few lines before and passed to
the stream opening functions.
The xmalloc function is written in a way that either 'st' is allocated
valid memory or xmalloc already dies.
The function calls to open_istream_* do not change 'st', as the pointer is
passed by reference and not a pointer of a pointer.

Hence 'st' cannot be null at that part of the code.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>