pgsql: Fix buffer allocations in encoding conversion routines so that

Log Message:
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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.

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic:
cyrillic_and_mic.c (r1.6.4.3 -> r1.6.4.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic/cyrillic_and_mic.c?r1=1.6.4.3&r2=1.6.4.4)
pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis:
euc_jp_and_sjis.c (r1.6.4.4 -> r1.6.4.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis/euc_jp_and_sjis.c?r1=1.6.4.4&r2=1.6.4.5)
pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5:
euc_tw_and_big5.c (r1.6.4.2 -> r1.6.4.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5/euc_tw_and_big5.c?r1=1.6.4.2&r2=1.6.4.3)
pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250:
latin2_and_win1250.c (r1.6.4.2 -> r1.6.4.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250/latin2_and_win1250.c?r1=1.6.4.2&r2=1.6.4.3)