chpst: fix undefined operation on sequence point.

Based on patch from

https://salsa.debian.org/debian/runit/-/blob/77a49befd7c1a08bfe0613454e540562529811fa/debian/patches/0016-fix-warning-for-undefined-operation-on-sequence-poin.patch

Author: ericonr

src/chpst.c

@@ -308,7 +308,8 @@ int main(int argc, const char **argv) {
     case 'n':
       switch (*optarg) {
         case '-':
-          if (optarg[scan_ulong(++optarg, &ul)]) usage(); nicelvl =ul;
+          ++optarg;
+          if (optarg[scan_ulong(optarg, &ul)]) usage(); nicelvl =ul;
           nicelvl *=-1;
           break;
         case '+': ++optarg;