Crash with struct/union combo in leaf definition

The following code causes a stack overflow and subsequent crash:

``` python
from bcc import BPF

text = """
struct routing_key_2 {
u32 key_field_0;
};
enum routing_actions_1 {
routing_nop,
};
struct routing_value_3 {
enum routing_actions_1 action;
union {
struct {
} nop;
} u;
};
BPF_TABLE("hash", struct routing_key_2, struct routing_value_3, routing, 512);
"""

b = BPF(text=text)
```

Start by running the Python reproducer in the issue and inspect the BCC handling of embedded BPF type definitions, especially the struct/union combination. Done means this input no longer causes a stack overflow or crash, while existing BPF table behavior remains intact.

Written by the indexing model from the issue text.