helpers.c: Handle msgpack str/bin objects with length 0 correctly

When converting a msgpack object to a String object, strings (and byte
arrays) with length 0 are handled as errors. This is fixed by
always using the msgpack data pointer as a valid pointer. For a NULL
pointer there is nothing to copy.

Test by @snoe

Fixes #3844

1 files changed, 3 insertions, 4 deletions

diff --git a/src/nvim/msgpack_rpc/helpers.c b/src/nvim/msgpack_rpc/helpers.c
index a119c4653a..5ef81721d4 100644
--- a/src/nvim/msgpack_rpc/helpers.c
+++ b/src/nvim/msgpack_rpc/helpers.c
@@ -94,10 +94,9 @@ bool msgpack_rpc_to_string(msgpack_object *obj, String *arg)
   FUNC_ATTR_NONNULL_ALL
 {
   if (obj->type == MSGPACK_OBJECT_BIN || obj->type == MSGPACK_OBJECT_STR) {
-    if (obj->via.bin.ptr == NULL) {
-      return false;
-    }
-    arg->data = xmemdupz(obj->via.bin.ptr, obj->via.bin.size);
+    arg->data = obj->via.bin.ptr != NULL
+                    ? xmemdupz(obj->via.bin.ptr, obj->via.bin.size)
+                    : NULL;
     arg->size = obj->via.bin.size;
     return true;
   }