Avoid failure when `format-analysis' stumbles upon unbound variables.

* module/language/tree-il/analyze.scm (proc-ref?): Wrap `variable-ref'
  in `false-if-exception'.

* test-suite/tests/tree-il.test ("warnings")["format"]("non-literal
  format string with forward declaration"): New test.

module/language/tree-il/analyze.scm

@@ -1354,7 +1354,8 @@ resort, return #t when EXP refers to the global variable SPECIAL-NAME."
     (($ <toplevel-ref> _ name)
      (let ((var (false-if-exception (module-variable env name))))
        (if var
-           (eq? (variable-ref var) proc)
+           (eq? (false-if-exception (variable-ref var)) ; VAR may be unbound
+                proc)
            (eq? name special-name))))      ; special hack to support local aliases
     (($ <module-ref> _ module name public?)
      (let ((m (false-if-exception (if public?

test-suite/tests/tree-il.test

@@ -2184,6 +2184,19 @@
                           #:opts %opts-w-format
                           #:to 'assembly)))))
 
+     (pass-if "non-literal format string with forward declaration"
+       (let ((w (call-with-warnings
+                 (lambda ()
+                   (compile '(begin
+                               (define (foo)
+                                 (format #t (_ "~A ~A!") "hello" "world"))
+                               (define _ bar))
+                            #:opts %opts-w-format
+                            #:to 'assembly)))))
+         (and (= (length w) 1)
+              (number? (string-contains (car w)
+                                        "non-literal format string")))))
+
      (pass-if "wrong format string"
        (let ((w (call-with-warnings
                  (lambda ()