TextualAdapter builds incorrect 0 and negative numbers

[aarrtteemm]

the next tests fail:

>>> from construct import *

from construct.text import *
TextualIntAdapter(Field("textintadapter", 3)).build(-12)
'12-'
TextualIntAdapter(Field("textintadapter", 1)).build(0)
Traceback (most recent call last):
File "", line 1, in
File "construct/core.py", line 206, in build
self.build_stream(obj, stream)
File "construct/core.py", line 214, in build_stream
self._build(obj, stream, Container())
File "construct/core.py", line 284, in _build
self.subcon._build(self._encode(obj, context), stream, context)
File "construct/core.py", line 324, in _build
_write_stream(stream, self.length, obj)
File "construct/core.py", line 306, in _write_stream
raise FieldError("expected %d, found %d" % (length, len(data)))
construct.core.FieldError: expected 1, found 0