hush: fix segfault on syntax error

Hush segfaults if it sees a syntax error while attempting to parse a
command:

 $ ./u-boot -c "'"
 ...
 syntax error
 Segmentation fault (core dumped)

This is due to a NULL pointer dereference of in_str->p in static_peek().
The problem is that the exit condition for the loop in
parse_stream_outer() checks for rcode not being -1, but rcode is only
ever 0 or 1.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Simon Glass <sjg@chromium.org)>
Tested-by: Simon Glass <sjg@chromium.org)>