x509_crt: Zero-initialize mbedtls_x509_time at declaration

[kraj]

'mbedtls_x509_time now' is a local struct variable. passing an uninitialized local as a const *
argument is UB-risk, since the callee is not
allowed to write into it.

Clang-21 got stricter about const pointer arguments finds it and flags it.

zero-initializing ensures all fields are defined.

Description

Fix build with clang-21

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