mbedtls_asn1_get_bitstring_null: fix rejection of short inputs
mbedtls_asn1_get_bitstring_null: fix rejection of short inputs

Fix improper rejection of bitstrings with length less than 2.

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