NativeCrypto: handle X.509 serial number 0

There is no need to try to convert numbers that are of zero length
because they'll be zero anyway. Returning NULL in this instance caused a
crash in X.509 code since it immediately tried to pass it to BigInteger
which through a NullPointerException.

(cherry picked from commit adadc611f606e693ed30b2e495326cb32968153d)

Bug: 9297758
Change-Id: I719ca7b8f086937ee25094bfe981987def855744
1 file changed