Fixed a signed-unsigned comparison in spatial_db.cc -- issue #293

Summary:
Fixed a signed-unsigned comparison in spatial_db.cc

utilities/spatialdb/spatial_db.cc:542:38: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
cc1plus: all warnings being treated as errors
make: *** [utilities/spatialdb/spatial_db.o] Error 1

Test Plan:
make spatial_db_test
./spatial_db_test

Reviewers: ljin, sdong, reddragon, igor

Reviewed By: reddragon

Subscribers: reddragon, leveldb

Differential Revision: https://reviews.facebook.net/D23565