file: reject paths using embedded %00

Mostly because we use C strings and they end at a binary zero so we know
we can't open a file name using an embedded binary zero.

Reported-by:  <research@g0blin.co.uk>

lib/file.c

@@ -196,8 +196,9 @@ static CURLcode file_connect(struct connectdata *conn, bool *done)
   int i;
   char *actual_path;
 #endif
+  int real_path_len;
 
-  real_path = curl_easy_unescape(data, data->state.path, 0, NULL);
+  real_path = curl_easy_unescape(data, data->state.path, 0, &real_path_len);
   if(!real_path)
     return CURLE_OUT_OF_MEMORY;
 
@@ -222,16 +223,23 @@ static CURLcode file_connect(struct connectdata *conn, bool *done)
      (actual_path[2] == ':' || actual_path[2] == '|')) {
     actual_path[2] = ':';
     actual_path++;
+    real_path_len--;
   }
 
   /* change path separators from '/' to '\\' for DOS, Windows and OS/2 */
-  for(i=0; actual_path[i] != '\0'; ++i)
+  for(i=0; i < real_path_len; ++i)
     if(actual_path[i] == '/')
       actual_path[i] = '\\';
+    else if(!actual_path[i]) /* binary zero */
+      return CURLE_URL_MALFORMAT;
 
   fd = open_readonly(actual_path, O_RDONLY|O_BINARY);
   file->path = actual_path;
 #else
+  if(memchr(real_path, 0, real_path_len))
+    /* binary zeroes indicate foul play */
+    return CURLE_URL_MALFORMAT;
+
   fd = open_readonly(real_path, O_RDONLY);
   file->path = real_path;
 #endif