[lldb] [Process/NetBSD] Fix handling LLDB_INVALID_SIGNAL_NUMBER

Fix NativeProcessNetBSD::Resume() to handle LLDB_INVALID_SIGNAL_NUMBER
correctly.  Fixes breakage caused by r372090 and r372300.  I have major
rewrite of that function pending; however, the fixes to gdb-remote
were committed prior to that.

llvm-svn: 372755