Fix segfault when passing invalid include path in fbtrace config via fbtracemgr

[Noremos]

Using the include directive allows you to insert configuration from a separate file. However, passing an invalid path causes the server to crash (for user sessions only). Using recursive include also leads to a crash

Steps to reproduce

1. create user NT password '123';
2. Create a file /tmp/trace_include_missing.conf with the following contents:

database
{
	enabled = true
}
include /a/b/c/d

3. Execute:
   ./fbtracemgr -se <server ip/port>:service_mgr -user NT -password 123 -start -name nytrace -config /tmp/trace_include_missing.conf

After this, a segfault will occur.