canbus: isotp: Fix NULL deref after overflow

The implementation checks if there are not enough buffers
to add the received data. However, a return was missing
after the error signaling.

Fixes #22657
Coverity-CID: 208191

Signed-off-by: Alexander Wachter <alexander@wachter.cloud>

subsys/canbus/isotp/isotp.c

@@ -432,6 +432,7 @@ static inline void receive_add_mem(struct isotp_recv_ctx *ctx, u8_t *data,
 	if (!ctx->act_frag) {
 		LOG_ERR("No fragmet left to append data");
 		receive_report_error(ctx, ISOTP_N_BUFFER_OVERFLW);
+		return;
 	}
 
 	net_buf_add_mem(ctx->act_frag, data + tailroom, len - tailroom);