pgsql: Fix buffer allocations in encoding conversion routines so that

Log Message:
-----------
Fix buffer allocations in encoding conversion routines so that they won't
fail on zero-length inputs. This isn't an issue in normal use because the
conversion infrastructure skips calling the converters for empty strings.
However a problem was created by yesterday's patch to check whether the
right conversion function is supplied in CREATE CONVERSION. The most
future-proof fix seems to be to make the converters safe for this corner case.

Modified Files:
--------------
pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic:
cyrillic_and_mic.c (r1.19 -> r1.20)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic/cyrillic_and_mic.c?r1=1.19&r2=1.20)
pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis:
euc_jp_and_sjis.c (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis/euc_jp_and_sjis.c?r1=1.21&r2=1.22)
pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5:
euc_tw_and_big5.c (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5/euc_tw_and_big5.c?r1=1.20&r2=1.21)
pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250:
latin2_and_win1250.c (r1.17 -> r1.18)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250/latin2_and_win1250.c?r1=1.17&r2=1.18)