Enum options treat empty string as 0

Take rpl_semi_sync_master_wait_point for example.
It is a Sys_var_enum with CMD_LINE(REQUIRED_ARG) and defaults to AFTER_COMMIT.

This quick test:

--source include/not_embedded.inc

SELECT @@GLOBAL.rpl_semi_sync_master_wait_point;

Results:

• AFTER_COMMIT with an empty .opt file
• argument parsing problem with --rpl-semi-sync-master-wait-point
• AFTER_SYNC with --rpl-semi-sync-master-wait-point= (note the dangling = with no text)
  • expected: invalid argument error
• invalid argument error with --rpl-semi-sync-master-wait-point=asdf
• AFTER_SYNC with --rpl-semi-sync-master-wait-point=0 as that is the 1st choice

Analysis: mysys/my_getopt.c § case GET_ENUM rejects extraneous text (e.g., =asdf) after the number (e.g., =0), but does not look for the existence of the number itself (=).