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/sql/unireg.cc b/sql/unireg.cc
index 083960523c1..92949931f77 100644
--- a/sql/unireg.cc
+++ b/sql/unireg.cc
@@ -921,8 +921,11 @@ static bool pack_fields(uchar **buff_arg, List<Create_field> &create_fields,
     it.rewind();
     while ((field=it++))
     {
-      memcpy(buff, field->comment.str, field->comment.length);
-      buff+= field->comment.length;
+      if (size_t l= field->comment.length)
+      {
+        memcpy(buff, field->comment.str, l);
+        buff+= l;
+      }
     }
   }
   *buff_arg= buff;