Bug#48327: Some crashes specific to FreeBSD ("embedded")

Bug#47139: Test "merge" crashes in "embedded" run

Backport patch for Bug#47139

storage/myisam/mi_dynrec.c

@@ -66,9 +66,12 @@ static int _mi_cmp_buffer(File file, const uchar *buff, my_off_t filepos,
 my_bool mi_dynmap_file(MI_INFO *info, my_off_t size)
 {
   DBUG_ENTER("mi_dynmap_file");
-  if (size > (my_off_t) (~((size_t) 0)))
+  if (size == 0 || size > (my_off_t) (~((size_t) 0)))
   {
+    if (size)
       DBUG_PRINT("warning", ("File is too large for mmap"));
+    else
+      DBUG_PRINT("warning", ("Do not mmap zero-length"));
     DBUG_RETURN(1);
   }
   /*