Fix for CONC-510: CoreDump using set env MARIADB_PLUGIN_DIR

The macro CLEAR_CLIENT_ERROR needs to check if mysql->net.extension
was already allocated to prevent access of uninitialized memory.

include/mysql.h

@@ -148,6 +148,7 @@ extern const char *SQLSTATE_UNKNOWN;
     (a)->net.last_errno= 0;\
     strcpy((a)->net.sqlstate, "00000");\
     (a)->net.last_error[0]= '\0';\
+    if ((a)->net.extension)\
       (a)->net.extension->extended_errno= 0;\
   } while (0)