[SPARK-19500] [SQL] Fix off-by-one bug in BytesToBytesMap

## What changes were proposed in this pull request?

Radix sort require that half of array as free (as temporary space), so we use 0.5 as the scale factor to make sure that BytesToBytesMap will not have more items than 1/2 of capacity. Turned out this is not true, the current implementation of append() could leave 1 more item than the threshold (1/2 of capacity) in the array, which break the requirement of radix sort (fail the assert in 2.2, or fail to insert into InMemorySorter in 2.1).

This PR fix the off-by-one bug in BytesToBytesMap.

This PR also fix a bug that the array will never grow if it fail to grow once (stay as initial capacity), introduced by #15722 .

## How was this patch tested?

Added regression test.

Author: Davies Liu <davies@databricks.com>

Closes #16844 from davies/off_by_one.