Correctly catch errors in running SMT solver

`dec_solve` was looking for a negative return code to indicate failure instead a non-zero return code. On Mac OS, I see a return code of 127 when the solver is not installed.

Author: SaswatPadhi

src/solvers/smt2/smt2_dec.cpp

@@ -121,7 +121,7 @@ decision_proceduret::resultt smt2_dect::dec_solve()
   int res =
     run(argv[0], argv, stdin_filename, temp_file_stdout(), temp_file_stderr());
 
-  if(res<0)
+  if(res != 0)
   {
     messaget log{message_handler};
     log.error() << "error running SMT2 solver" << messaget::eom;