pgsql: Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^

Log Message:
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Have numeric 0 ^ 4.3 return 1, rather than an error, and have 0 ^ 0.0
return 1, rather than error.

This was already the float8 behavior.

Modified Files:
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pgsql/src/backend/utils/adt:
numeric.c (r1.110 -> r1.111)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/numeric.c?r1=1.110&r2=1.111)