Send qXfer:traceframe-info:read when traceframe is selected.

Send qXfer:traceframe-info:read when traceframe is selected.

When I do 'si', I find many 'qXfer:traceframe-info:read' packets are sent,
which is not necessary.  It slows down the single step.
(gdb) si
Sending packet: $qTStatus#49...Packet received: T0;tnotrun:0;tframes:0;tcreated:0;tfree:500000;tsize:500000;circular:0;disconn:0;starttime:0;stoptime:0;username:;notes::
Sending packet: $Z0,80483c7,1#b4...Packet received: OK
Sending packet: $Z0,4ce5b6b0,1#6e...Packet received: OK
Sending packet: $QPassSignals:e;10;14;17;1a;1b;1c;21;24;25;2c;4c;#5f...Packet received: OK
Sending packet: $vCont;s:p1b15.1b15;c#20...Packet received: T0505:44efffbf;04:44efffbf;08:d1830408;thread:p1b15.1b15;core:3;
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $mbfffef40,40#c0...Packet received: d183040878efffbf2e840408030000000000a040030000000500000070efffbf07000000010000004984040807000000030000000500000000000000b396e84c
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $z0,80483c7,1#d4...Packet received: OK
Sending packet: $z0,4ce5b6b0,1#8e...Packet received: OK
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01
Sending packet: $qXfer:traceframe-info:read::0,fff#0b...Packet received: E01

This problem was introduced by this patch
(https://sourceware.org/ml/gdb-patches/2013-04/msg00000.html), in
which get_traceframe_number is not checked before calling
traceframe_available_memory.  This patch moves the check to
remote_traceframe_info, say, if GDB doesn't have traceframe selected, GDB
doesn't need to send qXfer:traceframe-info:read packets.

With this patch applied, there is no qXfer:traceframe-info:read sent
out and single step is speed up a little bit.

Here is the experiment I did:

	   Num of single step	Original	Patched

single-step cpu_time 10000	8.08		7.57
single-step cpu_time 20000	16.23		14.23
single-step cpu_time 30000	24.19		21.59
single-step cpu_time 40000	32.49		28.0
single-step wall_time 10000	14.1974210739	13.2641420364
single-step wall_time 20000	28.5278921127	25.0541369915
single-step wall_time 30000	42.5864038467	38.0038759708
single-step wall_time 40000	57.2107698917	49.2350611687
single-step vmsize 10000	16128		16388
single-step vmsize 20000	16128		16388
single-step vmsize 30000	16260		16520
single-step vmsize 40000	16444		16704

The patch is tested on x86_64-linux.

gdb:

2013-10-24  Yao Qi  <yao@codesourcery.com>

	* remote.c (remote_traceframe_info): Return early if
	traceframe is not selected.