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>

llvm-svn: 213829