Fix over-allocation of space for array_out()'s result string.

Fix over-allocation of space for array_out()'s result string.

array_out overestimated the space needed for its output, possibly by
a very substantial amount if the array is multi-dimensional, because
of wrong order of operations in the loop that counts the number of
curly-brace pairs needed.  While the output string is normally
short-lived, this could still cause problems in extreme cases.

An additional minor error was that it counted one more delimiter than
is actually needed.

Repair those errors, add an Assert that the space is now correctly
calculated, and make some minor improvements in the comments.

I also failed to resist the temptation to get rid of an integer
modulus operation per array element; a simple comparison is sufficient.

This bug dates clear back to Berkeley days, so back-patch to all
supported versions.

Keiichi Hirobe, minor additional work by me

Discussion: https://postgr.es/m/CAH=EFxE9W0tRvQkixR2XJRRCToUYUEDkJZk6tnADXugPBRdcdg@mail.gmail.com