sql: POW pegs the CPU

[knz]

SELECT power(56051.85009165843::decimal, 98.23741371063426::decimal)

found with RSG
https://teamcity.cockroachdb.com/viewLog.html?buildId=52805&buildTypeId=Cockroach_Nightlies_AllNightlyTests

[knz]

Another repro in:
https://teamcity.cockroachdb.com/viewLog.html?buildId=67103&buildTypeId=Cockroach_Nightlies_AllNightlyTests

panic: func exec timeout: SELECT pow(2.306257620454719e+12::decimal, 49.18687811476825::decimal)