Add missing array-enlargement logic to test_regex.c.
The stanza to report a "partial" match could overrun the initially
allocated output array, so it needs its own copy of the array-resizing
logic that's in the main loop. I overlooked the need for this in
ca8217c10.
Per report from Alexander Lakhin.
Discussion: https://postgr.es/m/
3206aace-50db-e02a-bbea-
76d5cdaa2cb6@gmail.com