Merge #587: Make randomization of a non-signing context a noop

6198375 Make randomization of a non-signing context a noop (Tim Ruffing)

Pull request description:

  Before this commit secp256k1_context_randomize called illegal_callback
  when called on a context not initialized for signing. This is not
  documented. Moreover, it is not desirable because non-signing contexts
  may use randomization in the future.

  This commit makes secp256k1_context_randomize a noop in this case. This
  is safe because the context cannot be used for signing anyway.

  This fixes #573 and it fixes rust-bitcoin/rust-secp256k1#82.

Tree-SHA512: 34ddfeb004d9da8f4a77c739fa2110544c28939378e779226da52f410a0e36b3aacb3ebd2e3f3918832a9027684c161789cfdc27a133f2f0e0f1c47e8363029c