Fix zero-length struct/union test. Remove nonsensical test.

Fix zero-length struct/union test. Remove nonsensical test.

The comment suggests this was meant to detect unions, but in fact it
compared f->c, the union/struct size, against f->next->c, the first
element's offset.

This affected only zero-length structs/unions with a first (zero-length)
element, as in this code:

    struct u2 {
    };

    struct u {
      struct u2 u2;
    } u;

    struct u f(struct u x)
    {
      return x;
    }

However, such structures turned out to be broken anyway, as code like this
was generated for the above f:

0000000000000000 <f>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 81 ec 10 00 00 00    sub    $0x10,%rsp
   b:   66 0f d6 45 f8          movq   %xmm0,-0x8(%rbp)
  10:   66 0f 6e 45 f8          movd   -0x8(%rbp),%xmm0
  15:   e9 00 00 00 00          jmpq   1a <f+0x1a>
  1a:   c9                      leaveq
  1b:   c3                      retq