Fix a memory read bug in lldb-server

NativeProcessProtocol::ReadMemoryWithoutTrap had a bug, where it failed
to properly remove inserted breakpoint opcodes if the memory read
partially overlapped the trap opcode. This could not happen on x86
because it has a one-byte breakpoint instruction, but it could happen on
arm, which has a 4-byte breakpoint instruction (in arm mode).

Since triggerring this condition would only be possible on an arm
machine (and even then it would be a bit tricky). I test this using a
NativeProcessProtocol unit test.

llvm-svn: 343076