zdb: fencepost error at zdb_cb.zcb_embedded_histogram[][]

Erroneous access detected by gcc UndefinedBehaviorSanitizer:
zdb.c:2424:7: runtime error: index 112 out of bounds for type 'uint64_t [112]'

Fix: increase histogram size by 1 to accommodate all possible sizes.

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4934
Issue #4883