Fix a bug in 'generate-new-buffer-name'

* src/buffer.c (Fgenerate_new_buffer_name): Test IGNORE for being
nil before calling string-equal, since the latter will compare
"nil and 'nil' as equal.  (Bug#27966)

* test/src/buffer-tests.el
(test-generate-new-buffer-name-bug27966): New test.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 649ddbe1839..0d0f43e937b 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1077,7 +1077,8 @@ is first appended to NAME, to speed up finding a non-existent buffer.  */)
 
   CHECK_STRING (name);
 
-  if (!NILP (Fstring_equal (name, ignore)) || NILP (Fget_buffer (name)))
+  if ((!NILP (ignore) && !NILP (Fstring_equal (name, ignore)))
+      || NILP (Fget_buffer (name)))
     return name;
 
   if (SREF (name, 0) != ' ') /* See bug#1229.  */