Fix inverted NOCOPY encode/decode parameter (bug#40407)

In {encode,decode}-coding-string, the NOCOPY parameter had the
opposite effect to what was intended and documented.  This 18 year old
bug (introduced in 4031e2bf0a) only affected calls with CODING-SYSTEM
being nil.

* src/coding.c (code_convert_string): Correct use of NOCOPY.
* test/src/coding-tests.el (coding-nocopy-trivial): New test.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/coding.c b/src/coding.c
index f0fc37dbdfa..1049f1b755a 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -9485,7 +9485,7 @@ code_convert_string (Lisp_Object string, Lisp_Object coding_system,
       if (! norecord)
         Vlast_coding_system_used = Qno_conversion;
       if (NILP (dst_object))
-        return (nocopy ? Fcopy_sequence (string) : string);
+        return nocopy ? string : Fcopy_sequence (string);
     }
 
   if (NILP (coding_system))