Do not read past the end of fmt_desc[]

Do not read past the end of fmt_desc[]

Callers of parse_format() pass a size in bytes while the parse_format()
function itself expects a number of elements. Fix this by making the
fmt_desc[] array NULL-terminated. This patch fixes the following Coverity
complaint:

CID 300986 (#1 of 1): Out-of-bounds access (OVERRUN)
overrun-buffer-arg: Overrunning array fmt_desc of 1 24-byte elements by
passing it to a function which accesses it at element index 23 (byte
offset 575) using argument 24U.

Cc: Roman Pen <r.peniaev@gmail.com>
Fixes: 634bd210c17a ("lib/pattern: add set of functions to parse combined pattern input")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>