gdbstub: avoid possible NULL pointer dereference

Coverity reports that s->chr is checked after put_packet dereferences it.
Move the check earlier, consistent with the code used for user-mode
emulation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

gdbstub.c

@@ -1443,15 +1443,17 @@ void gdb_exit(CPUArchState *env, int code)
   if (gdbserver_fd < 0 || s->fd < 0) {
       return;
   }
+#else
+  if (!s->chr) {
+      return;
+  }
 #endif
 
   snprintf(buf, sizeof(buf), "W%02x", (uint8_t)code);
   put_packet(s, buf);
 
 #ifndef CONFIG_USER_ONLY
-  if (s->chr) {
-      qemu_chr_delete(s->chr);
-  }
+  qemu_chr_delete(s->chr);
 #endif
 }