Fix some warnings and a buffer overrun in wtperf:
wtperf.c:411: valgrind: Invalid write of size 1
wtperf.c:301:12: error: conversion to 'uint64_t' from 'int' may change the sign of the result
wtperf.c:736:20: error: conversion to 'uint32_t' from 'int' may change the sign of the result