buffer: readInt32 avoids readUInt32

Fixes GH #7809

Test: test/simple/test-readint.js
The extraneous parentheses have been removed.

On a side note, the equivalent  lines in readUInt32 are

    val = val + (buffer[offset] << 24 >>> 0);

https://github.com/joyent/node/blob/v0.10/lib/buffer.js#L628

    val = val + (buffer[offset + 3] << 24 >>> 0);

https://github.com/joyent/node/blob/v0.10/lib/buffer.js#L633

It might be simpler for these to use the assignment operators (e.g. `val += buffer[offset] << 24 >>> 0;`)