record-btrace: fix assertion when enabling recording after re-run

Reading symbols from /bin/true...(no debugging symbols found)...done.
(gdb) b _start
Function "_start" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (_start) pending.
(gdb) r
Starting program: /bin/true

Breakpoint 1, 0x00000039a0400af0 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) rec b
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /bin/true

Breakpoint 1, 0x00000039a0400af0 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) rec b
gdb/record-btrace.c:154: internal-error: record_btrace_open:
 Assertion `record_btrace_thread_observer == NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)


gdb/
	* record-btrace.c (record_btrace_close): Call
	record_btrace_auto_disable.

testsuite/
	* gdb.btrace/enable.exp: Add regression test.