sh: fix bug in parameter substitution (with shortest path removal)

Avoid a core dump on the following snippet:
  V="a.b"; V=${V%.*}

toys/pending/sh.c

@@ -2122,7 +2122,7 @@ barf:
             }
 
             if (yy != -1) {
-              if (*delete && (*delete)->arg==ifs) ifs[yy] = 0;
+              if (delete && *delete && (*delete)->arg==ifs) ifs[yy] = 0;
               else push_arg(delete, ifs = xstrndup(ifs, yy));
             }
           }