hash hash with an empty __VA_ARGS__ in a macro

Example code:
```
#define CAT(a, ...)        a##__VA_ARGS__
#define X(a, ...)          CAT(a)
#define LEVEL_2            (2)

X(LEVEL_2)
```
gcc output:
```
(2)
```

simplecpp output:
```
test2.c:33: syntax error: failed to expand 'X', Invalid ## usage when expanding 'CAT': Unexpected token ')'
```
