secp256k1/src/tests.c: Properly handle sscanf return value

Summary:
This pull request fixes a bug which allows the sh variable to be used uninitialized when sscanf returns EOF.

Signed-off-by: Mustapha Abiola <mustapha@trilemma.net>

This is a backport of secp256k1 PR650

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5356