Fix unitialized variable in coverage_log()

When providing the ability to force coverage printouts to occur, some
code was moved around that allowed the "hash" variable to be used
unitialized.  This fixes that.

Thanks to Ben for pointing out the problem.

Bug #1577

lib/coverage.c

@@ -134,8 +134,8 @@ coverage_log(enum vlog_level level, bool suppress_dups)
         return;
     }
 
+    hash = coverage_hash();
     if (suppress_dups) {
-        hash = coverage_hash();
         if (coverage_hit(hash)) {
             VLOG(level, "Skipping details of duplicate event coverage for "
                  "hash=%08"PRIx32" in epoch %u", hash, epoch);