Fix missing error check on BIO_get_mem_ptr()

[ndossche]

LibreSSL docs state this can fail. OpenSSL has no docs for this, but the implementation goes via BIO_ctrl() which can fail (e.g. via a callback). Example Valgrind trace:

Invalid read of size 1
  memmove (at /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
  memcpy (string_fortified.h:29)
  ossl_str_new (ossl.c:107)
  ossl_membio2str (ossl_bio.c:36)
  ossl_x509ext_get_value (ossl_x509ext.c:390)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  rb_catch_obj (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  rb_catch_obj (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_vm_invoke_proc (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_proc_call_kw (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  ruby_run_node (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  (below main) (libc_start_call_main.h:58)

This was found by a hybrid static-dynamic analyser that looks for inconsistent handling of error checks in bindings.

[rhenium]

It seems OpenSSL documented this, too, actually:

BIO_set_mem_eof_return(), BIO_set_mem_buf() and BIO_get_mem_ptr() return 1 on success or a value which is less than or equal to 0 if an error occurred.

I don't expect it to actually fail with an unmodified OpenSSL, as the BIO shouldn't have a callback attached, but the return value should still be checked.