rsp: Return proper error condition on unrecognized 'v' command

[stffrdhrn]

An upstream commit 57809e5e5 in gdb causes gdb remote client to abort if the server returns a non blank response when handling an unknown request.

gdb remote connect fails with: Remote replied unexpectedly to 'vMustReplyEmpty': E01

This change is to return "" is as per spec so gdb connections do not abort.

• ChangeLog:

20156-09-18 Stafford Horne shorne@gmail.com

* debug/rsp-server.c: return "" on unknown vpacket

[jeremybennett]

Thanks for fixing this

[stffrdhrn]

Thanks for reviewing