ftp: truncate first segment if over max length

The first segment was not limited to the configured maximum line length
allowing it to be up to 65k. This could result in the next input length
being negative, which while handled properly by the code, did trigger a
debug validation assertion.

The fix is to be consistent and apply the limit to the first segment as
well, which does ensure the input_len could never be less than 0.

Ticket #5281

src/app-layer-ftp.c

@@ -387,6 +387,9 @@ static int FTPGetLineForDirection(FtpState *state, FtpLineState *line_state)
         }
     }
 
+    /* Should be guaranteed by the caller. */
+    DEBUG_VALIDATE_BUG_ON(state->input_len <= 0);
+
     uint8_t *lf_idx = memchr(state->input, 0x0a, state->input_len);
 
     if (lf_idx == NULL) {
@@ -397,13 +400,18 @@ static int FTPGetLineForDirection(FtpState *state, FtpLineState *line_state)
          * if we see fragmentation then it's definitely something you
          * should alert about */
         if (line_state->current_line_db == 0) {
-            line_state->db = FTPMalloc(state->input_len);
+            int32_t input_len = state->input_len;
+            if ((uint32_t)input_len > ftp_max_line_len) {
+                input_len = ftp_max_line_len;
+                state->current_line_truncated = true;
+            }
+            line_state->db = FTPMalloc(input_len);
             if (line_state->db == NULL) {
                 return -1;
             }
             line_state->current_line_db = 1;
-            memcpy(line_state->db, state->input, state->input_len);
+            memcpy(line_state->db, state->input, input_len);
-            line_state->db_len = state->input_len;
+            line_state->db_len = input_len;
         } else if (!state->current_line_truncated) {
             int32_t input_len = state->input_len;
             if (line_state->db_len + input_len > ftp_max_line_len) {