Fix an off by one in `rb_ary_resize` #567

Job	Run time
omnibus compilations, trigger	1s
omnibus compilations, #1	28m 11s
omnibus compilations, #5	25m 51s
omnibus compilations, #4	26m 34s
omnibus compilations, #3	25m 36s
omnibus compilations, #2	30m 21s
omnibus compilations, #6	26m 29s
omnibus compilations, #7	26m 11s
omnibus compilations, #8	26m 48s
omnibus compilations, #9	27m 2s
omnibus compilations, #11	26m 25s
omnibus compilations, #12	24m 31s
omnibus compilations, #10	27m 26s
omnibus compilations, result	1s
	5h 21m 27s

Provide feedback