Bug #14399795 : ISSUES RELATED TO SETTING AUDIT_LOG_STRATEGY

DURING SERVER STARTUP

The options parser now correctly checks for ambiguous prefixes in 
enumerated variables and emits an error when the value supplied is
ambiguous.

No test added since mysql-test-run.pl can't handle server startup 
failures as an expected state.

mysys/my_getopt.c

@@ -710,6 +710,11 @@ static int setval(const struct my_option *opts, void *value, char *argument,
           }
           *(ulong*)value= arg;
         }
+        else if (type < 0)
+        {
+          res= EXIT_AMBIGUOUS_OPTION;
+          goto ret;
+        }
         else
           *(ulong*)value= type - 1;
       }