Fix crash when connection was null

Fix crash when connection was null

JIRA - https://code.sec.samsung.net/jira/browse/TSIX-6919

callstack was as following.

Callstack Information (PID:11475)
Call Stack Count: 24
 0: gsignal + 0x9c (0xb63d3470) [/usr/lib/libc-2.30.so] + 0x30470
 1: abort + 0x10c (0xb63bd780) [/usr/lib/libc-2.30.so] + 0x1a780
 2: _dbus_abort + 0x68 (0xa6a356e0) [/usr/lib/libdbus-1.so.3.19.11] + 0x396e0
 3: _dbus_warn_check_failed + 0x88 (0xa6a2a128) [/usr/lib/libdbus-1.so.3.19.11] + 0x2e128
 4: _dbus_warn_return_if_fail + 0x30 (0xa6a2a830) [/usr/lib/libdbus-1.so.3.19.11] + 0x2e830
 5: dbus_connection_send_with_reply_and_block + 0x214 (0xa6a0ef8c) [/usr/lib/libdbus-1.so.3.19.11] + 0x12f8c
 6: stt_dbus_request_hello + 0x58 (0xa6ab8c75) [/usr/lib/libstt.so] + 0xdc75

Change-Id: I182bda3b921a63a6c43916fe1acb968fb3ebe505