stream_buffer: reject message length truncation at runtime in writer

Defect: prvWriteMessageToBuffer() can write a truncated message-length header
into a message buffer when the data length does not fit the configured
configMESSAGE_BUFFER_LENGTH_TYPE and configASSERT() is compiled out,
desynchronizing framing at the receiver.

Root cause: the writer stores the length as a configMESSAGE_BUFFER_LENGTH_TYPE
value xMessageLength and only guards the "fits without truncation" condition
with configASSERT( ( size_t ) xMessageLength == xDataLengthBytes ). The
subsequent write was gated solely on available space, so with asserts disabled
a length that overflows the length type is written truncated while the full
data is copied, corrupting the message framing seen by the receiver.

Fix: gate the length-plus-data write on
( ( size_t ) xMessageLength == xDataLengthBytes ) in addition to the space
check. When the length cannot be represented without truncation, take the
"not enough space" path and write nothing. This runtime check is the only guard
against truncation when asserts are disabled.

A host regression test kept outside this repository demonstrates the fault
before the change and its absence afterwards (red then green).

stream_buffer.c

@@ -1065,16 +1065,21 @@ static size_t prvWriteMessageToBuffer( StreamBuffer_t * const pxStreamBuffer,
         /* Ensure the data length given fits within configMESSAGE_BUFFER_LENGTH_TYPE. */
         configASSERT( ( size_t ) xMessageLength == xDataLengthBytes );
 
-        if( xSpace >= xRequiredSpace )
+        if( ( ( size_t ) xMessageLength == xDataLengthBytes ) && ( xSpace >= xRequiredSpace ) )
         {
-            /* There is enough space to write both the message length and the message
+            /* The message length fits within configMESSAGE_BUFFER_LENGTH_TYPE and
+             * there is enough space to write both the message length and the message
              * itself into the buffer.  Start by writing the length of the data, the data
              * itself will be written later in this function. */
             xNextHead = prvWriteBytesToBuffer( pxStreamBuffer, ( const uint8_t * ) &( xMessageLength ), sbBYTES_TO_STORE_MESSAGE_LENGTH, xNextHead );
         }
         else
         {
-            /* Not enough space, so do not write data to the buffer. */
+            /* Either there is not enough space, or the message length cannot be
+             * represented by configMESSAGE_BUFFER_LENGTH_TYPE without truncation.
+             * Writing a truncated length header would corrupt the message
+             * framing at the receiver, so do not write any data to the buffer.  When asserts are
+             * disabled this runtime check is the only guard against truncation. */
             xDataLengthBytes = 0;
         }
     }