gdb: Fix an infrun debug log message

Run the test gdb.threads/multiple-successive-infcall.exp by hand, if
you turn on 'debug infrun 1', you'll see that the debug line fixed in
this commit is printed and contains the wrong $pc value.  Fixed in
this commit.

gdb/ChangeLog:

	* infrun.c (do_target_wait): Change old version of $pc printed.

gdb/infrun.c

@@ -3533,7 +3533,7 @@ do_target_wait (ptid_t ptid, struct target_waitstatus *status, int options)
 	    fprintf_unfiltered (gdb_stdlog,
 				"infrun: PC of %s changed.  was=%s, now=%s\n",
 				target_pid_to_str (tp->ptid),
-				paddress (gdbarch, tp->prev_pc),
+				paddress (gdbarch, tp->suspend.stop_pc),
 				paddress (gdbarch, pc));
 	  discard = 1;
 	}