Fix crash when sending Gnus message (Bug#39207)

* src/alloc.c (resize_string_data): The string must be multibyte.
When not bothering to reallocate, do bother to change the byte count.
* test/src/alloc-tests.el (aset-nbytes-change) New test.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/src/alloc-tests.el b/test/src/alloc-tests.el
index 4eb776a0555..aa1ab1648f8 100644
--- a/test/src/alloc-tests.el
+++ b/test/src/alloc-tests.el
@@ -51,3 +51,10 @@
     (should-not (eq x y))
     (dotimes (i 4)
       (should (eql (aref x i) (aref y i))))))
+
+;; Bug#39207
+(ert-deftest aset-nbytes-change ()
+  (let ((s (make-string 1 ?a)))
+    (dolist (c (list 10003 ?b 128 ?c ?d (max-char) ?e))
+      (aset s 0 c)
+      (should (equal s (make-string 1 c))))))