psa_asymmetric_sign: consistently fill unused output with '!'
psa_asymmetric_sign: consistently fill unused output with '!'

Fill the unused part of the output buffer with '!', for consistency
with hash and mac.

On error, set the output length to the output buffer size and fill the
output buffer with '!', again for consistency with hash and mac. This
way an invalid output is more visible in a memory dump.

Restructure the error paths so that there is a single place where the
unused part of the output buffer is filled.

Also remove a redundant initialization of *signature_length to 0.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls