ci: murdock: fix output error on missing builddir after failed build

.murdock

@@ -96,7 +96,7 @@ compile() {
         fi
     fi
 
-    echo "-- build directory size: $(du -sh ${BINDIR} | cut -f1)"
+    test -d ${BINDIR} && echo "-- build directory size: $(du -sh ${BINDIR} | cut -f1)"
 
     # cleanup
     rm -Rf build