read-print: Correctly read "(. x)".

* guix/read-print.scm (read-with-comments): Check whether REST is a pair
before calling 'set-cdr!'.
* tests/read-print.scm ("read-with-comments: half dot notation"): New test.

guix/read-print.scm

@@ -179,9 +179,11 @@ BLANK-LINE? is true, assume PORT is at the beginning of a new line."
       (match lst
         (() result)
         (((? dot?) . rest)
-         (let ((dotted (reverse rest)))
-           (set-cdr! (last-pair dotted) (car result))
-           dotted))
+         (if (pair? rest)
+             (let ((dotted (reverse rest)))
+               (set-cdr! (last-pair dotted) (car result))
+               dotted)
+             (car result)))
         ((x . rest) (loop (cons x result) rest)))))
 
   (let loop ((blank-line? blank-line?)

tests/read-print.scm

@@ -58,6 +58,11 @@ expressions."
   (call-with-input-string "(a . b)"
     read-with-comments))
 
+(test-equal "read-with-comments: half dot notation"
+  '(lambda x x)
+  (call-with-input-string "(lambda (. x) x)"
+    read-with-comments))
+
 (test-equal "read-with-comments: list with blank line"
   `(list with ,(vertical-space 1) blank line)
   (call-with-input-string "\