Fix #27 (#29)

The assertion check was placed incorrectly because tx_mailbox may be 0xFF if there are no free mailboxes available.
This is not a fault condition.

Author: pavel-kirienko

stm32/libcanard/bxcan/src/bxcan.c

@@ -578,7 +578,7 @@ bool bxCANPush(const uint8_t     iface_index,      //
             {
                 // All TX mailboxes are busy (this is highly unlikely); at the same time we know that there is no
                 // higher or equal priority frame that is currently pending. Therefore, priority inversion has just
-                // happend (sic!), because we can't enqueue the higher priority frame due to all TX mailboxes being
+                // happened (sic!), because we can't enqueue the higher priority frame due to all TX mailboxes being
                 // busy. This scenario is extremely unlikely, because in order for it to happen, the application would
                 // need to transmit 4 (four) or more CAN frames with different CAN ID ordered from high ID to low ID
                 // nearly at the same time. For example:
@@ -600,16 +600,13 @@ bool bxCANPush(const uint8_t     iface_index,      //
         {
             tx_mailbox = 0U;
         }
-
-        BXCAN_ASSERT(tx_mailbox < 3U);  // Index check - the value must be correct here
     }
 
     // By this time we've proven that:
     // - The input is valid.                                                        (input_ok    == true)
     // - There are no mailboxes with priority >= the priority of the new frame.     (prio_higher == false)
     // - A priority inversion would not occur.                                      (tx_mailbox  != 0xFF)
-    // - A free TX mailbox is available.                                            (tx_mailbox   < 3U)
-    // Therefore it is safe to enqueue the frame now.
+    // Therefore it is safe to enqueue the frame now if a free mailbox is available.
     if (input_ok && !prio_higher && (tx_mailbox < 3U))
     {
         volatile BxCANTxMailboxType* const mb = &bxcan_base->TxMailbox[tx_mailbox];