[lldb] [Process/NetBSD] Fix handling piod_len from PT_IO calls

[mgorny]

Summary

Fix bugs in piod_len return value processing in ReadMemory()
and WriteMemory() methods. In particular, add support for piod_len == 0
indicating EOF, and fix summing bytes_read/bytes_written when PT_IO does
partial reads/writes.

The EOF condition could happen if LLDB attempts to read past
vm.maxaddress, e.g. as a result of RBP containing large (invalid) value.
Previously, the 0 return caused the function to retry reading via PT_IO
indefinitely, effectively deadlooping lldb-server.

Partial reads probably did not occur in practice, yet they would cause
ReadMemory() to return incorrect bytes_read and/or overwrite previously
read data.

WriteMemory() suffered from analoguous problems.

[krytarowski]

I think there is a bug thought in the code.

1. bytes_read should return the bytes transferred from the function, it returns the value from the previous iteration only
2. bytes_read != 0 should be replaced with io.piod_len != 0

[mgorny]

Also included fixes for WriteMemory().

lldb/source/Plugins/Process/NetBSD/NativeProcessNetBSD.cpp

do {
     io.piod_addr = dst + bytes_read;
 
     Status error = NativeProcessNetBSD::PtraceWrapper(PT_IO, GetID(), &io);
     if (error.Fail())
       return error;
 
     bytes_read = io.piod_len;
     io.piod_len = size - bytes_read;
-  } while (bytes_read < size);
+  } while (bytes_read < size && bytes_read != 0);
 
   return Status();
 }
 
 Status NativeProcessNetBSD::WriteMemory(lldb::addr_t addr, const void *buf,
                                         size_t size, size_t &bytes_written) {
   const unsigned char *src = static_cast<const unsigned char *>(buf);
   Status error;