llext: fix minor Coverity issue #434586 (again)

The check combination of "is zero" and then "less than zero" leaves open
the theoretical possibility of a positive return value, which would
continue on with an uinitialized 'sym'.

Checking for "not zero" has the same effect and covers all situations.

Commit 56fd8544 did the same thing for 'rela' a few lines above, but
missed this instance. No other places in the llext code have this issue.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>