ssl_build_cert_chain() may crash on NULL SSL argument

[DDvO]

The recently merged #11457 introduced in ssl/ssl_cert.c the call

    xs_ctx = X509_STORE_CTX_new_with_libctx(s->ctx->libctx, s->ctx->propq);

which crashes if s = NULL.
This happens for instance when called by ssl3_ctx_ctrl() in ssl/s3_lib.c as follows.

    return ssl_build_cert_chain(NULL, ctx, larg);

[DDvO]

Here is a backtrace I got while doing tests for #11470.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff74cc094 in ssl_build_cert_chain (s=0x0, ctx=0x61b000001580, flags=3) at ssl/ssl_cert.c:875
875	    xs_ctx = X509_STORE_CTX_new_with_libctx(s->ctx->libctx, s->ctx->propq);
(gdb) bt
#0  0x00007ffff74cc094 in ssl_build_cert_chain (s=0x0, ctx=0x61b000001580, flags=3) at ssl/ssl_cert.c:875
#1  0x00007ffff74be8eb in ssl3_ctx_ctrl (ctx=0x61b000001580, cmd=105, larg=3, parg=0x0) at ssl/s3_lib.c:3955
#2  0x00007ffff74e7fe9 in SSL_CTX_ctrl (ctx=0x61b000001580, cmd=105, larg=3, parg=0x0) at ssl/ssl_lib.c:2533
#3  0x0000000000437766 in setup_ssl_ctx (ctx=0x615000000d00, e=0x0) at apps/cmp.c:1813
#4  0x000000000043b2b1 in setup_client_ctx (ctx=0x615000000d00, e=0x0) at apps/cmp.c:2357
#5  0x000000000043fb5a in cmp_main (argc=3, argv=0x7fffffffdd10) at apps/cmp.c:3265
#6  0x0000000000469344 in do_cmd (prog=0x60f0000004f0, argc=3, argv=0x7fffffffdd10) at apps/openssl.c:490
#7  0x0000000000468593 in main (argc=3, argv=0x7fffffffdd10) at apps/openssl.c:302

[t8m]

This is already fixed by 64e54bf