Fail early if argc <= 0 or argv[0] is NULL

Under musl optind will be 1 if getopt is called with argc == 0.
Under glibc it is not quite clear what will happen and I haven't tested
it.

In either case, this triggers a bug where argv ends up pointing at
envp and argc is set to -1 which has further unclear implications on the
execution of the program.

By failing early, these issues can be safely avoided.

fputs and exit are used instead of errx as errx does not have the
information necessary to print a meaningful program name.

doas.c

@@ -260,6 +260,11 @@ main(int argc, char **argv)
 	const char *cwd;
 	char **envp;
 
+	if (argc <= 0 || argv == NULL || argv[0] == NULL) {
+		fprintf(stderr, "doas: executed without argv\n");
+		exit(1);
+	}
+
 	setprogname("doas");
 
 	closefrom(STDERR_FILENO + 1);