linux-record: Fix bad fall-through for pipe/pipe2

linux-record: Fix bad fall-through for pipe/pipe2

This patch added handling for some syscalls to linux-record.c:

  https://sourceware.org/ml/gdb-patches/2015-10/msg00452.html

But for both `pipe' and `pipe2' the patch lacks a statement after an
`if', such that the following `break' is interpreted as the `if'-body
instead.

This adds the missing (return-) statements for the conditionals.

gdb/ChangeLog:

	* linux-record.c (record_linux_system_call): Add missing return
	statements to handling of pipe and pipe2 syscalls.