Fix broken assertion for rows read

We also can have rows read on an EOF packet. In fact, that's very common
since the end of a SELECT query is always marked with an EOF, not with
an OK packet.

This would crash MySQL when building with assertions enabled (which
isn't what we do on release builds, which is why this isn't blowing up
in prod today).

Author: dbussink

sql/sql_error.h

@@ -393,7 +393,7 @@ class Diagnostics_area {
   }
 
   ulonglong rows_read() const {
-    assert(m_status == DA_OK);
+    assert(m_status == DA_OK || m_status == DA_EOF);
     return m_rows_read;
   }