Enable -Wconditional-uninitialized to warn on potentially uninitialized reads.
Fix the sole such warning in Bitcoin Core in GetRdRand(): r1 would be
set to 0 on rdrand failure, so initializing it to 0 is a non-functional
From "Intel 64 and IA-32 ArchitecturesSoftware Developer's Manual" ,
page 1711: "CF=1 indicates that the data in the destination is valid.
Otherwise CF=0 and the data in the destination operand will be returned
as zeros for the specified width."
This is a backport of Core PR18843