gdbstub: don't fail on vCont; C04:0; c packets

The thread-id of 0 means any CPU but we then ignore the fact we find
the first_cpu in this case who can have an index of 0. Instead of
bailing out just test if we have managed to match up thread-id to a
CPU.

Otherwise you get:
  gdb_handle_packet: command='vCont;C04:0;c'
  put_packet: reply='E22'

The actual reason for gdb sending vCont;C04:0;c was fixed in a
previous commit where we ensure the first_cpu's tid is correctly
reported to gdb however we should still behave correctly next time it
does send 0.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>

Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

gdbstub.c

@@ -938,23 +938,16 @@ static int gdb_handle_vcont(GDBState *s, const char *p)
             if (res) {
                 goto out;
             }
-            idx = tmp;
+
             /* 0 means any thread, so we pick the first valid CPU */
-            if (!idx) {
-                idx = cpu_gdb_index(first_cpu);
-            }
+            cpu = tmp ? find_cpu(tmp) : first_cpu;
 
-            /*
-             * If we are in user mode, the thread specified is actually a
-             * thread id, and not an index. We need to find the actual
-             * CPU first, and only then we can use its index.
-             */
-            cpu = find_cpu(idx);
             /* invalid CPU/thread specified */
-            if (!idx || !cpu) {
+            if (!cpu) {
                 res = -EINVAL;
                 goto out;
             }
+
             /* only use if no previous match occourred */
             if (newstates[cpu->cpu_index] == 1) {
                 newstates[cpu->cpu_index] = cur_action;