Cast the NULL argument of string_concat to unsigned char *.

string_concat reads the args with va_arg(ap, const unsigned char *),
and the NULL macro may have the wrong type (e.g. int).

Many places pass string literals of type char * to string_concat.
This is in principle also a violation, but I'm ignoring it for now
because if it becomes a problem with some C implementation, then so
will the use of unsigned char * with printf "%s", which is so
widespread in ELinks that I'm not going to try fixing it now.

src/main/version.c

@@ -123,7 +123,7 @@ get_dyn_full_version(struct terminal *term, int more)
 		comma, "UTF-8",
 #endif
 		comma,
-		NULL
+		(unsigned char *) NULL
 	);
 
 	add_modules_to_string(&string, term);

src/protocol/file/file.c

@@ -124,7 +124,8 @@ add_dir_entry(struct directory_entry *entry, struct string *page,
 
 	if (entry->attrib[0] == 'd' && *dircolor) {
 		/* The <b> is for the case when use_document_colors is off. */
-		string_concat(page, "<font color=\"", dircolor, "\"><b>", NULL);
+		string_concat(page, "<font color=\"", dircolor, "\"><b>",
+			      (unsigned char *) NULL);
 	}
 
 	add_string_to_string(page, &html_encoded_name);

src/protocol/proxy.c

@@ -60,7 +60,8 @@ proxy_uri(struct uri *uri, unsigned char *proxy,
 	struct string string;
 
 	if (init_string(&string)
-	    && string_concat(&string, "proxy://", proxy, "/", NULL)
+	    && string_concat(&string, "proxy://", proxy, "/",
+			     (unsigned char *) NULL)
 	    && add_uri_to_string(&string, uri, URI_BASE)) {
 		/* There is no need to use URI_BASE when calling get_uri()
 		 * because URI_BASE should not add any fragments in the first

src/util/string.h

@@ -167,7 +167,8 @@ struct string *add_string_to_string(struct string *to, const struct string *from
 struct string *add_file_to_string(struct string *string, const unsigned char *filename);
 struct string *add_crlf_to_string(struct string *string);
 
-/* Adds each C string to @string until a terminating NULL is met. */
+/* Adds each C string to @string until a terminating
+ * (unsigned char *) NULL is met. */
 struct string *string_concat(struct string *string, ...);
 
 /* Extends the string with @times number of @character. */