FreeRTOS_TCP_Transmission_IPv4,6(): Move deref behind nullptr check (#1137)

Fix a possible null pointer derefernce:
The pointer returned by pxDuplicateNetworkBufferWithDescriptor()
was being dereferenced outside its null checks.
This could only happen if ipconfigZERO_COPY_TX_DRIVER was enabled.

Conveniently, the statement that used the pointer could be moved
into the scope of the null check.

Co-authored-by: Andreas Nordal <andreas.nordal@zivid.com>

Author: anordal

source/FreeRTOS_TCP_Transmission_IPv4.c

@@ -141,17 +141,17 @@ void prvTCPReturnPacket_IPV4( FreeRTOS_Socket_t * pxSocket,
         }
         #endif /* ipconfigZERO_COPY_TX_DRIVER */
 
-        /* MISRA Ref 11.3.1 [Misaligned access] */
-        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
-        /* coverity[misra_c_2012_rule_11_3_violation] */
-        pxIPHeader = ( ( IPHeader_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
-
         #ifndef __COVERITY__
             if( pxNetworkBuffer != NULL ) /* LCOV_EXCL_BR_LINE the 2nd branch will never be reached */
         #endif
         {
             NetworkInterface_t * pxInterface;
 
+            /* MISRA Ref 11.3.1 [Misaligned access] */
+            /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
+            /* coverity[misra_c_2012_rule_11_3_violation] */
+            pxIPHeader = ( ( IPHeader_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
+
             /* Map the Ethernet buffer onto a TCPPacket_t struct for easy access to the fields. */
 
             /* MISRA Ref 11.3.1 [Misaligned access] */

source/FreeRTOS_TCP_Transmission_IPv6.c

@@ -147,19 +147,20 @@ void prvTCPReturnPacket_IPV6( FreeRTOS_Socket_t * pxSocket,
         }
         #endif /* ipconfigZERO_COPY_TX_DRIVER */
 
-        configASSERT( pxNetworkBuffer->pucEthernetBuffer != NULL );
-
-        /* MISRA Ref 11.3.1 [Misaligned access] */
-        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
-        /* coverity[misra_c_2012_rule_11_3_violation] */
-        pxIPHeader = ( ( IPHeader_IPv6_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
-
         #ifndef __COVERITY__
             if( pxNetworkBuffer != NULL ) /* LCOV_EXCL_BR_LINE the 2nd branch will never be reached */
         #endif
         {
             eARPLookupResult_t eResult;
             NetworkInterface_t * pxInterface;
+
+            configASSERT( pxNetworkBuffer->pucEthernetBuffer != NULL );
+
+            /* MISRA Ref 11.3.1 [Misaligned access] */
+            /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
+            /* coverity[misra_c_2012_rule_11_3_violation] */
+            pxIPHeader = ( ( IPHeader_IPv6_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
+
             /* Map the Ethernet buffer onto a TCPPacket_t struct for easy access to the fields. */
 
             /* MISRA Ref 11.3.1 [Misaligned access] */