buffer: truncate buffer after string decode

When our estimates for a storage size are higher than the actual length
of decoded data, the destination buffer should be truncated. Otherwise
`Buffer::Length` will give misleading information to C++ layer.

fix #7365

Signed-off-by: Fedor Indutny <fedor@indutny.com>
buffer: return uint if MSB is 1 in readUInt32

Fix issue where a signed integer is returned.

Example:

var b = new Buffer(4);
b.writeUInt32BE(0xffffffff);
b.readUInt32BE(0) == -1

Signed-off-by: Trevor Norris <trev.norris@gmail.com>