client: fix UBSAN error in failover calculation
client: fix UBSAN error in failover calculation

A misplaced typecast caused the following:

/data/somelongdirectorytoavoidrpathissues/src/kudu/src/kudu/client/scanner-internal.cc:433:72:
runtime error: unsigned integer overflow: 4 - 5 cannot be represented in type 'unsigned long'

This patch simply rearranges the expression a bit to avoid underflow
possibilities.

Change-Id: Icd137f86443bc4af59f8a4290a45151afda6c090
Reviewed-on: http://gerrit.cloudera.org:8080/9132
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>
Reviewed-by: Dan Burkert <2591e5f46f28d303f9dc027d475a5c60d8dea17a@cloudera.com>

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