[lldb] [Process/NetBSD] Fix segfault when handling watchpoint

Fix the watchpoint/breakpoint code to search for matching thread entry
in m_threads explicitly rather than assuming that it will be present
at specified index.  The previous code segfault since it wrongly assumed
that the index will match LWP ID which was incorrect even for a single
thread (where index was 0 and LWP ID was 1).

While fixing that off-by-one error would help for this specific task,
I believe it is better to be explicit in what we are searching for.

Differential Revision: https://reviews.llvm.org/D63791

llvm-svn: 364780