selftests/net: Ensure assert() triggers in psock_tpacket.c

The get_next_frame() function in psock_tpacket.c was missing a return statement in its default switch case, leading to a compiler warning.

This was caused by a bug_on(1) call, which is defined as an assert(), being compiled out because NDEBUG is defined during the build.

Instead of adding a return NULL; which would silently hide the error and could lead to crashes later, this change restores the original author's intent. By adding #undef NDEBUG before including <assert.h>, we ensure the assertion is active and will cause the test to abort if this unreachable code is ever executed.

---
 tools/testing/selftests/net/psock_tpacket.c | 1 +
 1 file changed, 1 insertion(+)

index 0dd909e325d9..2938045c5cf9 100644
@@ -22,6 +22,7 @@
  *   - TPACKET_V3: RX_RING
  */
 
+#undef NDEBUG
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>

2.50.1.703.g449372360f-goog