READ, #- or #+. and *read-suppress*; wrong offset after failed reader conditional

[Tobias C. Rittweiler]

When READ while *READ-SUPPRESS* is T encounters a reader conditional which succeeds,
the file-position afterwards is not the same as if *READ-SUPPRESS* was NIL.

This can make debugging awkward when you switch between *READ-SUPPRESS* values
for debugging purposes.

Test case:

;; Conditional succeeds
(defvar *str1*
"#-q 1 2")

;; Conditional succeeds
(defvar *str2*
"#-q
1
2")

;; Conditional succeeds
(defvar *str3*
"#+sbcl
1
2")

;; Conditional fails
(defvar *str4*
"#+q
1
2")

(defun test-read (string)
  (list
   :expected
   (with-input-from-string (s string)
     (let ((*read-suppress* nil))
       (read s))
     (file-position s))
   :read-supress
   (with-input-from-string (s string)
     (let ((*read-suppress* t))
       (read s))
     (file-position s))))

(values (test-read *str1*)
            (test-read *str2*)
            (test-read *str3*)
            (test-read *str*4))

evaluates to

(:EXPECTED 6 :READ-SUPRESS 5),
(:EXPECTED 6 :READ-SUPRESS 5),
(:EXPECTED 9 :READ-SUPRESS 8),
(:EXPECTED 8 :READ-SUPRESS 8)