Another chance for cutting back overhead due to C++ exceptions being
enabled; the `dict_sys_t` class is a good candidate because its
locking methods are called frequently.

Binary size reduction this time:

    text	  data	   bss	   dec	   hex	filename
 24448622	2436488	9473537	36358647	22ac9f7	build/release/sql/mariadbd
 24448474	2436488	9473601	36358563	22ac9a3	build/release/sql/mariadbd