src/dfa/dfa.h: simplify constructor to avoid g++-3.4 bug

src/dfa/dfa.h: simplify constructor to avoid g++-3.4 bug

On g++-3.4.6 re2c tests SIGSEGVed due to use of uninitialized data:

```
$ valgrind ... ./re2c -8 a.re -o foo.c
Conditional jump or move depends on uninitialised value(s)
   at 0x432F23: re2c::tcpool_t::insert(re2c::tcmd_t const*) (tcmd.cc:202)
   by 0x421FDA: re2c::freeze_tags(re2c::dfa_t&) (freeze.cc:45)
   by 0x43A7FF: re2c::ast_to_dfa(re2c::spec_t const&, re2c::Output&) (compile.cc:88)
   by 0x43B052: push_back (stl_iterator.h:614)
   by 0x43B052: re2c::compile(re2c::Scanner&, re2c::Output&, re2c::Opt&) (???:0)
   by 0x449D29: main (main.cc:31)
 Uninitialised value was created by a heap allocation
   at 0x403252F: operator new[](unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x42FC9E: re2c::find_state(re2c::determ_context_t&) (dfa.h:37)
   by 0x429BD9: re2c::dfa_t::dfa_t(re2c::nfa_t const&, re2c::opt_t const*, std::string const&, re2c::Warn&) (determinization.cc:56)
   by 0x43A76C: re2c::ast_to_dfa(re2c::spec_t const&, re2c::Output&) (compile.cc:69)
   by 0x43B052: push_back (stl_iterator.h:614)
   by 0x43B052: re2c::compile(re2c::Scanner&, re2c::Output&, re2c::Opt&) (???:0)
   by 0x449D29: main (main.cc:31)
```

the problem here arose in default array constructor:

```c++
     explicit dfa_state_t(size_t nchars)
         : // ...
         , tcmd(new tcmd_t*[nchars + 2]()) // +2 for final and fallback epsilon-transitions
         // ...
```

g++-3.4.6 can't figure out zero-initialization rule (likely a gcc bug).

The change uses non-initializing new[] and memset() instead.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>