gnu: guile-email: Fix build.

* gnu/packages/patches/guile-email-fix-tests.patch: New file.
* gnu/local.mk (DIST_PATCH_DATA): Add it.
* gnu/packages/guile-xyz.scm (guile-email): Use it.

gnu/local.mk

@@ -1206,6 +1206,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/guile-present-coding.patch		\
   %D%/packages/patches/guile-rsvg-pkgconfig.patch		\
   %D%/packages/patches/guile-emacs-fix-configure.patch		\
+  %D%/packages/patches/guile-email-fix-tests.patch		\
   %D%/packages/patches/guile-ssh-fix-test-suite.patch		\
   %D%/packages/patches/guile-ssh-read-error.patch		\
   %D%/packages/patches/gtk2-respect-GUIX_GTK2_PATH.patch	\

gnu/packages/guile-xyz.scm

@@ -1285,7 +1285,9 @@ tracker's SOAP service, such as @url{https://bugs.gnu.org}.")
              version ".tar.lz"))
        (sha256
         (base32
-         "1rc8r0fgvflnyq5ckl7ii8sghpsgpkzxa8vskjr1ak2kyar6m35k"))))
+         "1rc8r0fgvflnyq5ckl7ii8sghpsgpkzxa8vskjr1ak2kyar6m35k"))
+       (patches
+        (search-patches "guile-email-fix-tests.patch"))))
     (build-system gnu-build-system)
     (native-inputs
      `(("pkg-config" ,pkg-config)

gnu/packages/patches/guile-email-fix-tests.patch

@@ -0,0 +1,35 @@
+This patch has been proposed upstream:
+https://lists.systemreboot.net/guile-email/87mtnv1r2p.fsf@gnu.org/T/#u.
+
+From c21fe0f0e28b80b606973d3e372e2bc8528c9766 Mon Sep 17 00:00:00 2001
+From: Mathieu Othacehe <othacehe@gnu.org>
+Date: Wed, 29 Sep 2021 12:47:35 +0000
+Subject: [PATCH 1/1] email: Do not use an empty bytevector to test the
+ charset.
+
+Using an empty bytevector no longer throws an exception since this Guile
+commit: 5ea8c69e9153a970952bf6f0b32c4fad6a28e839.
+
+* email/email.scm (post-process-content-transfer-encoding): Use a bytevector
+containg the 'e' character to test the charset validity.
+---
+ email/email.scm | 3 ++-
+ 1 file changed, 2 insertions(+), 1 deletion(-)
+
+diff --git a/email/email.scm b/email/email.scm
+index 3f4e194..ac70463 100644
+--- a/email/email.scm
++++ b/email/email.scm
+@@ -832,7 +832,8 @@ values. The returned headers is a string and body is a bytevector."
+   (define (valid-charset? charset)
+     (catch #t
+       (lambda ()
+-        (bytevector->string (make-bytevector 0 0) charset)
++        ;; Try to convert a bytevector containg the 'e' character.
++        (bytevector->string (make-bytevector 1 48) charset)
+         #t)
+       (const #f)))
+ 
+-- 
+2.33.0
+