gum: fix ptrace error handling in thread modification protocol

[DoranekoSystems]

The thread modification code wasn't properly handling error cases during
ptrace operations, causing deadlocks. This fixes the communication protocol
in gum_linux_handle_modify_thread_comms to properly receive and handle
error acknowledgments from the child process.

See:

frida-gum/gum/backend-linux/gumprocess-linux.c

Lines 664 to 682 in 0671c27

	static gpointer
	gum_linux_handle_modify_thread_comms (gpointer data)
	{
	GumLinuxModifyThreadContext * ctx = data;
	gint fd = ctx->fd[0];
	gboolean success = FALSE;
	gum_put_ack (fd, GUM_ACK_READY);
	if (gum_await_ack (fd, GUM_ACK_READ_REGISTERS))
	{
	ctx->func (ctx->thread_id, &ctx->regs_data, ctx->user_data);
	gum_put_ack (fd, GUM_ACK_MODIFIED_REGISTERS);
	success = gum_await_ack (fd, GUM_ACK_WROTE_REGISTERS);
	}
	return GSIZE_TO_POINTER (success);
	}