modules/png: Read/write unknown chunk types properly

* modules/png/core/chunk.scm (chunk-type->vector): Return the type converted to
  a bytevector for unknown chunks.
* modules/png/fsm/chunk-context.scm (action:store-type): Bugfix: Read unknown
  chunk types "as is".

modules/png/core/chunk.scm

@@ -3,6 +3,7 @@
 
 (define-module (png core chunk)
   #:use-module (oop goops)
+  #:use-module (ice-9 iconv)
   #:use-module (png core common)
   #:use-module (png core crc)
   #:use-module (srfi srfi-1)
@@ -161,7 +162,10 @@ the list."
 
 (define-method (chunk-type->vector (type <symbol>))
   "Convert a PNG chunk TYPE to a vector.  Return the vector."
-  (cadr (png-chunk-type-info type)))
+  (let ((info (png-chunk-type-info type)))
+    (if info
+        (cadr (png-chunk-type-info type))
+        (string->bytevector (symbol->string type) "ASCII"))))
 
 (define-method (png-chunk-type-info (chunk <png-chunk>))
   (png-chunk-type-info (png-chunk-type chunk)))

modules/png/fsm/chunk-context.scm

@@ -1,5 +1,6 @@
 (define-module (png fsm chunk-context)
   #:use-module (ice-9 binary-ports)
+  #:use-module (ice-9 iconv)
   #:use-module (rnrs bytevectors)
   #:use-module (oop goops)
   #:use-module (png fsm context)
@@ -93,7 +94,15 @@
   (action:store ctx byte)
   (let ((chunk (context-result ctx))
         (data  (context-buffer ctx)))
-    (png-chunk-type-set! chunk (car (vector->chunk-type data)))
+    (let ((type (vector->chunk-type data)))
+      (if type
+          (png-chunk-type-set! chunk (car type))
+          (let ((type (string->symbol (bytevector->string data "ASCII"))))
+            (log-warning "action:store-type: Unknown chunk type: ~a (~a)"
+                         data
+                         type)
+            (png-chunk-type-set! chunk
+                                 type))))
     (%buffer-reset! ctx (png-chunk-length chunk)))
   ctx)