py/objstr: Protect against creating bytes(n) with n negative.

Prior to this patch uPy (on a 32-bit arch) would have severe issues when
calling bytes(-1): such a call would call vstr_init_len(vstr, -1) which
would then +1 on the len and call vstr_init(vstr, 0), which would then
round this up and allocate a small amount of memory for the vstr.  The
bytes constructor would then attempt to zero out all this memory, thinking
it had allocated 2^32-1 bytes.