imap-send: handle NULL return of next_arg()

imap-send: handle NULL return of next_arg()

next_arg() returns NULL if it runs out of arguments.  Most call sites
already handle that gracefully.  Check in the remaining cases as well.
Replace the NULL pointer with an empty string at the bottom of
get_cmd_result() -- it's nicely reported as an unexpected response a
few lines down.  Error out explicitly at the remaining sites.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>