TSDB query start time underflow

gistfile1.diff

diff --git a/src/core/TsdbQuery.java b/src/core/TsdbQuery.java
index 2edd53c..8776e89 100644
--- a/src/core/TsdbQuery.java
+++ b/src/core/TsdbQuery.java
@@ -401,7 +401,8 @@ final class TsdbQuery implements Query {
     // but this doesn't really matter.
     // Additionally, in case our sample_interval is large, we need to look
     // even further before/after, so use that too.
-    return getStartTime() - Const.MAX_TIMESPAN * 2 - sample_interval;
+    final long t = getStartTime() - Const.MAX_TIMESPAN * 2 - sample_interval;
+    return t > 0 ? t : 0;
   }
 
   /** Returns the UNIX timestamp at which we must stop scanning.  */