core: fix buffer overflow in function utf8_next_char and return NULL for empty string

Now the function utf8_next_char with an empty string returns NULL instead of
the next char, which is most of the time after an allocated buffer.

And the function utf8_char_size with an empty string now returns 0 instead of
1.

This indirectly fixes a buffer overflow in function eval_string_range_chars
when the input string is empty (for example when doing `/eval -n ${chars:}`).

src/plugins/alias/alias.c

@@ -420,6 +420,7 @@ void
 alias_hook_command (struct t_alias *alias)
 {
     char *str_priority_name, *str_completion;
+    const char *ptr_command;
 
     if (alias->hook)
     {
@@ -442,11 +443,10 @@ alias_hook_command (struct t_alias *alias)
     str_completion = NULL;
     if (!alias->completion)
     {
-        weechat_asprintf (
-            &str_completion,
-            "%%%%%s",
-            (weechat_string_is_command_char (alias->command)) ?
-            weechat_utf8_next_char (alias->command) : alias->command);
+        ptr_command = (weechat_string_is_command_char (alias->command)) ?
+            weechat_utf8_next_char (alias->command) : alias->command;
+        weechat_asprintf (&str_completion, "%%%%%s",
+                          (ptr_command) ? ptr_command : "");
     }
 
     alias->hook = weechat_hook_command (
@@ -681,11 +681,20 @@ alias_new (const char *name, const char *command, const char *completion)
     if (!command || !command[0])
         return NULL;
 
-    while (weechat_string_is_command_char (name))
+    while (name && weechat_string_is_command_char (name))
     {
         name = weechat_utf8_next_char (name);
     }
 
+    if (!name || !name[0])
+    {
+        weechat_printf (NULL,
+                        _("%s%s: invalid alias name: \"%s\""),
+                        weechat_prefix ("error"), ALIAS_PLUGIN_NAME,
+                        "");
+        return NULL;
+    }
+
     ptr_alias = alias_search (name);
     alias_free (ptr_alias);