ltlsmin: fix incorrect check for dlsym error

Fix #435 reported by Yann Thierry-Mieg.

* spot/ltsmin/spins_interface.cc (sym): Fix incorrect check introduced
by dc4a4771.
9 jobs for adl/435 in 148 minutes and 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed alpine-gcc #108043

01:39:30

passed arch-clang #108044

00:55:40

passed arch-gcc-glibcxxdebug #108045

02:28:06

passed debian-gcc-snapshot #108042

01:34:37

passed debian-stable-gcc #108040

01:02:59

passed debian-unstable-gcc-coverage #108041

01:51:36

90.6%
passed mingw-shared #108046

00:19:51

passed mingw-static #108047

00:10:47

passed raspbian #108048
armv7

01:31:54