MDEV-22387: Do not violate __attribute__((nonnull))

This follows up commit
commit 94a520ddbe39ae97de1135d98699cf2674e6b77e and
commit 7c5519c12d46ead947d341cbdcbb6fbbe4d4fe1b.

After these changes, the default test suites on a
cmake -DWITH_UBSAN=ON build no longer fail due to passing
null pointers as parameters that are declared to never be null,
but plenty of other runtime errors remain.

1 files changed, 5 insertions, 2 deletions

diff --git a/include/my_sys.h b/include/my_sys.h
index 08b06951d0d..ac1730eeaff 100644
--- a/include/my_sys.h
+++ b/include/my_sys.h
@@ -534,8 +534,11 @@ static inline int my_b_write(IO_CACHE *info, const uchar *Buffer, size_t Count)
 {
   if (info->write_pos + Count <= info->write_end)
   {
-    memcpy(info->write_pos, Buffer, Count);
-    info->write_pos+= Count;
+    if (Count)
+    {
+      memcpy(info->write_pos, Buffer, Count);
+      info->write_pos+= Count;
+    }
     return 0;
   }
   return _my_b_write(info, Buffer, Count);