The execution of the shebang script requires putting interpreter path,

possible option and script path in the place of argv[0] supplied to
execve(2).  It is possible and valid for the substitution to be shorter
then the argv[0].

Avoid signed underflow in this case.

Submitted by:	Devon H. O'Dell <devon.odell gmail com>
PR:	kern/155321
MFC after:	1 week

sys/kern/imgact_shell.c

@@ -195,7 +195,7 @@ exec_shell_imgact(imgp)
 	length = (imgp->args->argc == 0) ? 0 :
 	    strlen(imgp->args->begin_argv) + 1;		/* bytes to delete */
 
-	if (offset - length > imgp->args->stringspace) {
+	if (offset > imgp->args->stringspace + length) {
 		if (sname != NULL)
 			sbuf_delete(sname);
 		return (E2BIG);