y4m_input_fetch_frame: fix ubsan null/zero offset warning

reported under clang-13; use a while loop in file_read() to force a size
check before attempting to read. buf (aux_buf) may be may be null when
no conversion is necessary.

y4minput.c:29:43: runtime error: applying zero offset to null pointer

Bug: b/229626362
Change-Id: Ia3250d6ff9c325faf48eaa31f4399e20837f8f7b
1 file changed