Split up signing and verification initialization
Set precomputation table late and unset early.

Set the global pointer to the precomputation table only after initializing
it completely, and unset it before doing any uninitialization.

This causes fail-fast behavior in case of race conditions between
initialization and operations using it.
Merge pull request #60

c259a7c Set precomputation table late and unset early. (Pieter Wuille)
04e34d1 Split up signing and verification initialization (Pieter Wuille)