don't assume positive argc

handle a rare, but possible case of argc being 0, in which case argv[0]
would be null.

note that both POSIX and ISO C standard allow argc to be 0 and in
practice this can be triggered via calling `exec(3)` family of functions
with NULL as the first `argv`.

options.c

@@ -98,9 +98,6 @@ void parse_options(int argc, char **argv)
 	static opt_t _options;
 
 	options = &_options;
-	progname = strrchr(argv[0], '/');
-	progname = progname ? progname + 1 : argv[0];
-
 	_options.from_stdin = false;
 	_options.to_stdout = false;
 	_options.using_null = false;
@@ -125,6 +122,11 @@ void parse_options(int argc, char **argv)
 	_options.clean_cache = false;
 	_options.private_mode = false;
 
+	if (argc > 0) {
+		s = strrchr(argv[0], '/');
+		progname = s != NULL && s[1] != '\0' ? s + 1 : argv[0];
+	}
+
 	optparse_init(&op, argv);
 	while ((opt = optparse_long(&op, longopts, NULL)) != -1) {
 		for (n = 0; n < (int)ARRLEN(longopts); ++n) { /* clang-tidy finds some non-sensical branch and thinks optarg == NULL is possible */

util.c

@@ -28,7 +28,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-const char *progname;
+const char *progname = "nsxiv";
 
 void* emalloc(size_t size)
 {