BoringSSL: Keep static analyzers happier with X509_get0_pubkey_bitstr()

BoringSSL: Keep static analyzers happier with X509_get0_pubkey_bitstr()

While this function could return NULL if the parameter issued to it were
NULL, that does not really happen here. Anyway, since this can result in
a warning from a static analyzer that does can see the return NULL
without fully understanding what it means here, check the return value
explicitly against NULL to avoid false warnings.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>