Added null check in modules/ca/src/client/udpiiu.cpp to prevent possi…

…ble dereferencing of null pointer. Flagged as error by Codacy static code analysis.

Codacy link: https://app.codacy.com/gh/epics-base/epics-base/file/42103575495/issues/source?bid=16430872&fileBranchId=16430872#l950
Launchpad bug: https://bugs.launchpad.net/epics-base/+bug/1862916

LP: #1862916

modules/ca/src/client/udpiiu.cpp

@@ -946,7 +946,7 @@ bool udpiiu::pushDatagramMsg ( epicsGuard < epicsMutex > & guard,
 
     caHdr * pbufmsg = ( caHdr * ) &this->xmitBuf[this->nBytesInXmitBuf];
     *pbufmsg = msg;
-    if ( extsize ) {
+    if ( extsize && pExt ) {
         memcpy ( pbufmsg + 1, pExt, extsize );
         if ( extsize != alignedExtSize ) {
             char *pDest = (char *) ( pbufmsg + 1 );