Add debug asserts / sanity checks to
GDBRemoteRegisterContext::ReadRegisterBytes and
GDBRemoteRegisterContext::WriteRegisterBytes to ensure we don't try
to read/write off the end of the register buffer.  This should never
happen but we've had some target confusion in the past where it
did; adding the checks is prudent to avoid crashing here if it happens
again.

<rdar://problem/16450971> 
<rdar://problem/16458182>



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213829 91177308-0d34-0410-b5e6-96231b3b80d8