When passed an empty OID, mbedtls_oid_get_numeric_string would read one
byte from the zero-sized buffer and return an error code that depends on
its value. This is demonstrated by the test suite changes, which
check that an OID with length zero and an invalid buffer pointer does
not cause Mbed TLS to segfault.
Also check that second and subsequent subidentifiers are terminated, and
add a test case for that. Furthermore, stop relying on integer division
by 40, use the same loop for both the first and subsequent
subidentifiers, and add additional tests.
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
