epan: ensure that the result of ws_label_strcpy is terminated

Unless there is no available space, ensure that the label_str
passed into ws_label_strcpy is null terminated, in the cases
where the string to copy is the empty string, or begins with
invalid UTF-8.

Fix #18560. Fix #18551.

epan/strutil.c

@@ -815,6 +815,8 @@ ws_label_strcpy(char *label_str, size_t buf_size, size_t pos,
     ssize_t idx, src_len;
     ssize_t free_len;
 
+    label_str[pos] = '\0';
+
     idx = 0;
     src_len = strlen(str);
     free_len = buf_size - pos - 1;