fix memory leak in unbound-streamtcp when open_svr failed

fix memory leak in unbound-streamtcp when open_svr failed

==1927474== Memcheck, a memory error detector
==1927474== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==1927474== Using Valgrind-3.16.0 and LibVEX; rerun with -h for copyright info
==1927474== Command: unbound-streamtcp -f localhost
==1927474==
fatal: bad server specs 'localhost'
==1927474==
==1927474== HEAP SUMMARY:
==1927474==     in use at exit: 131,186 bytes in 4 blocks
==1927474==   total heap usage: 5 allocs, 1 frees, 132,210 bytes allocated
==1927474==
==1927474== 40 bytes in 1 blocks are still reachable in loss record 1 of 4
==1927474==    at 0x483F751: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1927474==    by 0x1E0573: sldns_buffer_new (sbuffer.c:21)
==1927474==    by 0x11ECED: send_em (streamtcp.c:374)
==1927474==    by 0x11E6C1: main (streamtcp.c:585)
==1927474==
==1927474== 40 bytes in 1 blocks are still reachable in loss record 2 of 4
==1927474==    at 0x483F751: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1927474==    by 0x1E0573: sldns_buffer_new (sbuffer.c:21)
==1927474==    by 0x11ECFA: send_em (streamtcp.c:375)
==1927474==    by 0x11E6C1: main (streamtcp.c:585)
==1927474==
==1927474== 65,553 bytes in 1 blocks are still reachable in loss record 3 of 4
==1927474==    at 0x483F751: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1927474==    by 0x1E0583: sldns_buffer_new (sbuffer.c:27)
==1927474==    by 0x11ECED: send_em (streamtcp.c:374)
==1927474==    by 0x11E6C1: main (streamtcp.c:585)
==1927474==
==1927474== 65,553 bytes in 1 blocks are still reachable in loss record 4 of 4
==1927474==    at 0x483F751: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1927474==    by 0x1E0583: sldns_buffer_new (sbuffer.c:27)
==1927474==    by 0x11ECFA: send_em (streamtcp.c:375)
==1927474==    by 0x11E6C1: main (streamtcp.c:585)
==1927474==
==1927474== LEAK SUMMARY:
==1927474==    definitely lost: 0 bytes in 0 blocks
==1927474==    indirectly lost: 0 bytes in 0 blocks
==1927474==      possibly lost: 0 bytes in 0 blocks
==1927474==    still reachable: 131,186 bytes in 4 blocks
==1927474==         suppressed: 0 bytes in 0 blocks
==1927474==
==1927474== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Signed-off-by: eaglegai <eaglegai@163.com>