lib: fix max size check in Buffer constructor

[bnoordhuis]

A logic error let buffer sizes larger than kMaxLength (0x3fffffff) slip
through. The .toString() operation (and possibly others as well) then
blew up because the buffer was larger than it could reasonably handle.

Fixes the following run-time assert:

#
# Fatal error in ../deps/v8/src/handles.h, line 48
# CHECK(location_ != NULL) failed
#

==== C stack trace ===============================

 1: V8_Fatal
 2: v8::String::NewFromUtf8(v8::Isolate*, char const*, v8::String::NewStringType, int)
 3: node::StringBytes::Encode(v8::Isolate*, char const*, unsigned long, node::encoding)
 4: node::Buffer::Utf8Slice(v8::FunctionCallbackInfo<v8::Value> const&)
 5: v8::internal::FunctionCallbackArguments::Call(void (*)(v8::FunctionCallbackInfo<v8::Value> const&))
 6: ??
 7: ??

Fixes: #649

R=@trevnorris?

[bnoordhuis]

The commit log is admittedly a little weak. The failing CHECK is related to but not exclusively caused by the bad length check in lib/buffer.js.

[trevnorris]

LGTM