Exercises the error path in DiffieHellman::ComputeSecret() in src/node_crypto.cc

Passing a bad key to DiffieHellman::ComputeSecret() made it zero the byte
before the heap allocated buffer due to an erroneous size calculation.

Fix a use-after-free bug and a memory leak in the error path of
DiffieHellman::ComputeSecret().

* the BIGNUM key was used after being freed with BN_free().
* the output buffer was not freed

`timezone` variable contains the difference, in seconds, between UTC and
local standard time (see `man 3 localtime` on Solaris).

Call to `tzset` is required to apply contents of `TZ` variable to
`timezone` variable.

BUG=v8:2064

Review URL: https://chromiumcodereview.appspot.com/10967066
Patch from Maciej Małecki <me@mmalecki.com>.

This is a back-port of upstream commit r12802.

The C standard allows plain `char` to be unsigned. The build environment
at Google trips this issue.

Ensure that uint8 values >= 128 are correctly promoted to int8 <= -1.