Steps to reproduce the problem
1. Set up replication (I used single master - single replica)
   Both have the suffix "dc=example,dc=com"
2. Create a sub suffix ou=sub,dc=example,dc=com only on master
3. Create an agreement to replicate ou=sub,dc=example,dc=com
4. Initialize the replica, which crashes the replica

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x438c1950 (LWP 27456)]
0x0000003c1c880f80 in strlen () from /lib64/libc.so.6
(gdb) bt
#0  0x0000003c1c880f80 in strlen () from /lib64/libc.so.6
#1  0x00000000006d3afe in slapi_mtn_set_state (sdn=0x1d62680, state=0x0)
    at ldap/servers/slapd/mapping_tree.c:3059
#2  0x00007f2c42084965 in repl_set_mtn_state_and_referrals (
    repl_root_sdn=0x1d62680, mtn_state=0x0, ruv=0x0, ruv_referrals=0x0,
    other_referrals=0x0) at ldap/servers/plugins/replication/replutil.c:764
#3  0x00007f2c42062495 in multimaster_extop_StartNSDS50ReplicationRequest (
    pb=0x1de1c20) at ldap/servers/plugins/replication/repl_extop.c:841
#4  0x00000000006e2211 in plugin_call_exop_plugins (pb=0x1de1c20, 
    oid=0x1c26eb0 "2.16.840.1.113730.3.5.3") at ldap/servers/slapd/plugin.c:393
#5  0x000000000041a453 in do_extended (pb=0x1de1c20)
    at ldap/servers/slapd/extendop.c:300
#6  0x00000000004127f3 in connection_dispatch_operation (conn=0x7f2c3f587728, 
    op=0x1de60d0, pb=0x1de1c20) at ldap/servers/slapd/connection.c:560
#7  0x0000000000413d18 in connection_threadmain ()
    at ldap/servers/slapd/connection.c:2161
#8  0x0000003c2c429aa3 in ?? () from /usr/lib64/libnspr4.so
#9  0x0000003c1d40729a in start_thread (arg=<value optimized out>)
    at pthread_create.c:297
#10 0x0000003c1c8e439d in clone () from /lib64/libc.so.6

Cause of the crase:
In the following code, slapi_mtn_get_state returns NULL since the mapping tree entry 'cn="ou=sub,dc=example,dc=com",cn=mapping tree,cn=config' does not exist on the Replica.  At the line 835, slapi_start_bulk_import fails due to the same reason.  In the error handling code, it tries to set the original state back even though the state does not exist.  We should skip resetting the mapping tree state.
(repl_extop.c)
 533 /*
 534  * This plugin entry point is called whenever a
 535  * StartNSDS50ReplicationRequest is received.
 536  */
 537 int
 538 multimaster_extop_StartNSDS50ReplicationRequest(Slapi_PBlock *pb)
 539 {
    [...]
 813     {
 814         char *mtnstate = slapi_mtn_get_state(repl_root_sdn);
 815         char **mtnreferral = slapi_mtn_get_referral(repl_root_sdn);
 816 
    [...]
 833         /* start the bulk import */
 834         slapi_pblock_set (pb, SLAPI_TARGET_DN, repl_root);
 835         rc = slapi_start_bulk_import (pb);
 836         if (rc != LDAP_SUCCESS)
 837         {
 838             response = NSDS50_REPL_INTERNAL_ERROR;
 839             /* reset the mapping tree state to what it was before
 840                we tried to do the bulk import */
 841             repl_set_mtn_state_and_referrals(repl_root_sdn, mtnstate,
 842                                              NULL, NULL, mtnreferral);