A nit to db_stress to terminate generated value at proper length

Summary: Will help while debugging if the generated value is truncated at proper length.

Test Plan: make db_stress;/db_stress --max_key=10000 --db=/tmp/mcr --threads=1 --ops_per_thread=10000

Reviewers: dhruba, vamsi

Reviewed By: vamsi

Differential Revision: https://reviews.facebook.net/D10845

tools/db_stress.cc

@@ -869,6 +869,7 @@ class StressTest {
     for (size_t i=sizeof(uint32_t); i < value_sz; i++) {
       v[i] = (char)(rand ^ i);
     }
+    v[value_sz] = '\0';
     return value_sz; // the size of the value set.
   }