Attached is a patch for two bugs in `imenu--generic-function'.  I ran
into them when using a function value for argument REGEXP, though
neither has to do particularly with that use case.  Instead, they both
have to do with the case where items are within comments or strings,
which might be skipped.

1. When checking whether the current item is inside a comment or string,
   it's necessary to first move point to position START.  Otherwise, the
   test is made at bol (which might not be within a comment or string,
   even though the START position is).

2. At the end, it's necessary to remove any empty menus that could have
   been added.  That can happen because of skipping things inside
   comments or strings.

In GNU Emacs 27.0.50 (build 3, x86_64-w64-mingw32)
 of 2018-03-21
Repository revision: e70d0c9e66d7a8609450b2889869d16aeb0363b5
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install -C 'CFLAGS=-O2 -static -g3''