Fix build on CentOS 6

Change-Id: Ibb0852760c7e1000aad822ee7063e35ff420b360
Reviewed-on: http://review.couchbase.org/112739
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>

src/bucketconfig/bc_file.cc

@@ -236,7 +236,7 @@ static std::string mkcachefile(const char *name, const char *bucket)
     bool is_dir = false;
     if (name != NULL) {
         buffer = std::string(name);
-        if (buffer.back() == '/') {
+        if (buffer.size() > 0 && buffer[buffer.size() - 1] == '/') {
             is_dir = true;
         }
     } else {