Fix a wrong return value in SEXP parser
Fix a wrong return value in SEXP parser

This function usually returns NULL, this looks like an oversight.
Addressing:
In file included from /home/jcerny/openscap/src/OVAL/probes/SEAP/sexp-parser.c:36:0:
/home/jcerny/openscap/src/OVAL/probes/SEAP/sexp-parser.c: In function ‘SEXP_parse’:
/home/jcerny/openscap/src/OVAL/probes/SEAP/_sexp-parser.h:140:27: warning: return makes pointer from integer without a cast [-Wint-conversion]
 #define SEXP_PRET_EUNDEF  255 /* unknown, unexpected error */
                           ^
/home/jcerny/openscap/src/OVAL/probes/SEAP/sexp-parser.c:1050:13: note: in expansion of macro ‘SEXP_PRET_EUNDEF’
      return SEXP_PRET_EUNDEF;
             ^~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/OVAL/probes/SEAP/_sexp-parser.h:140:27: warning: return makes pointer from integer without a cast [-Wint-conversion]
 #define SEXP_PRET_EUNDEF  255 /* unknown, unexpected error */
                           ^
/home/jcerny/openscap/src/OVAL/probes/SEAP/sexp-parser.c:1304:10: note: in expansion of macro ‘SEXP_PRET_EUNDEF’
   return SEXP_PRET_EUNDEF;
          ^~~~~~~~~~~~~~~~

OpenSCAP/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,jan-cerny/openscap,jan-cerny/openscap,mpreisler/openscap,OpenSCAP/openscap,OpenSCAP/openscap,mpreisler/openscap,OpenSCAP/openscap,redhatrises/openscap,mpreisler/openscap,jan-cerny/openscap,jan-cerny/openscap,mpreisler/openscap,redhatrises/openscap,jan-cerny/openscap,mpreisler/openscap,redhatrises/openscap,mpreisler/openscap,redhatrises/openscap,OpenSCAP/openscap,redhatrises/openscap