According to the SRFI-1 spec, 'length+' must be passed a proper or
circular list.  It should raise an error when passed a non-pair or an
improper list, but instead it returns #f in such cases:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (use-modules (srfi srfi-1))
scheme@(guile-user)> (length+ 5)
$1 = #f
scheme@(guile-user)> (length+ 'x)
$2 = #f
scheme@(guile-user)> (length+ '(x . y))
$3 = #f
--8<---------------cut here---------------end--------------->8---

One side effect of this is that SRFI-1 'map', which uses 'length+' to
validate the arguments and find the shortest length, accepts improper
lists and non-pairs as arguments as long as one of the arguments is a
proper list:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (map + '(1 2) '(1 2 3 . 4))
$4 = (2 4)
scheme@(guile-user)> (map + '() 2)
$5 = ()
scheme@(guile-user)> (map + '(1) 2)
ERROR: In procedure cdr:
ERROR: In procedure cdr: Wrong type (expecting pair): 2
--8<---------------cut here---------------end--------------->8---

The attached patch fixes these problems.

     Mark