stream-ssl: Fix crash on NULL private key and valid certificate.

stream-ssl: Fix crash on NULL private key and valid certificate.

Running ovsdb-server with empty private-key and non-empty certificate
(or otherwise) causes crash:

 # ovsdb-tool create ./etc/openvswitch/conf.db ./vswitch.ovsschema
 # ovsdb-server --remote=punix:./db.sock \
                --remote=db:Open_vSwitch,Open_vSwitch,manager_options \
                --private-key=db:Open_vSwitch,SSL,private_key \
                --certificate=db:Open_vSwitch,SSL,certificate \
                --bootstrap-ca-cert=db:Open_vSwitch,SSL,ca_cert

 # ovs-vsctl --no-wait init
 # ovs-vsctl --no-wait set-ssl pkey.key cert.cert ca.cert
 # ovs-vsctl --no-wait set SSL . private_key='""'
 # ovs-vsctl --no-wait set SSL . certificate='cert.new'

 ==25513==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000
 ==25513==The signal is caused by a READ memory access.
 ==25513==Hint: address points to the zero page.
    #0 0x7ff7582aa0a9 in __GI___strlen_sse2
    #1 0x7ff759bdde81  (/lib64/libasan.so.5+0xace81)
    #2 0x7ff759479932  (/lib64/libcrypto.so.1.1+0xb3932)
    #3 0x7ff759473c5a in BIO_ctrl (/lib64/libcrypto.so.1.1+0xadc5a)
    #4 0x7ff7598decc1 in SSL_CTX_use_certificate_file (/lib64/libssl.so.1.1+0x40cc1)
    #5 0x4dbaa7 in stream_ssl_set_certificate_file__ lib/stream-ssl.c:1170
    #6 0x4dca2e in stream_ssl_set_key_and_cert lib/stream-ssl.c:1216
    #7 0x4146b2 in reconfigure_ssl ovsdb/ovsdb-server.c:1254
    #8 0x409c83 in main ovsdb/ovsdb-server.c:368
    #9 0x7ff758233812 in __libc_start_main
    #10 0x40f6bd in _start (ovsdb-server+0x40f6bd)

 AddressSanitizer can not provide additional info.
 SUMMARY: AddressSanitizer: SEGV (/lib64/libc.so.6+0x9a0a9) in __GI___strlen_sse2
 ==25513==ABORTING

Another way to reproduce is to use non-initialized DB entry for
private-key and a file for certificate in ovsdb-server cmdline.

The root cause is that stream_ssl_set_key_and_cert() triggers
configuration for both key and cert if any of them is valid, keeping
it possible for one of them to be NULL.

Fixes: 6f1e91b1d7c0 ("stream-ssl: Make changing keys and certificate at runtime reliable.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Ben Pfaff <blp@ovn.org>