Fixed a compile error which tries to check whether a size_t < 0 in env_posix.cc

Summary:
Fixed a compile error which tries to check whether a size_t < 0 in env_posix.cc

util/env_posix.cc:180:16: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
    } while (r < 0 && errno == EINTR);
             ~ ^ ~
1 error generated.

Test Plan: make check all

Reviewers: igor, haobo

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17379