CLEANUP: stats: Avoid computation with uninitialized bits.

CLEANUP: stats: Avoid computation with uninitialized bits.

stats_map_lookup() sets bit SMP_F_CONST in the uninitialized member
flags of a stack-allocated sample, leaving the other bits
uninitialized.  All code paths that can access the struct only ever
check for this specific flag, so there is no risk of unintended
behavior.

Nevertheless fix it as it triggers warnings in static code analysis
tools and might become a problem on future revisions of the code.

Problem found in version 1.5.