Fix Coverity Scan warning on uninitialized variable

*** CID 157851:  Uninitialized members  (UNINIT_CTOR)
/forestdb/src/hbtrie.h: 158 in HBTrieValue::HBTrieValue(unsigned char,
void *, unsigned long)()
152                 valueLen = _value_len;
CID 157851:  Uninitialized members  (UNINIT_CTOR)
Non-static class member "offset" is not initialized in this
constructor nor in any functions that it calls.

Change-Id: I09c75d500663437862d549c5d127aea7400b41cb
Reviewed-on: http://review.couchbase.org/70878
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>

Author: hisundar

src/hbtrie.h

@@ -144,7 +144,7 @@ class HBTrieValue {
     HBTrieValue(uint8_t _flags,
                 void *_value,
                 size_t _value_len) :
-        flags(_flags), valueLen(sizeof(offset)), binary(nullptr)
+        flags(_flags), valueLen(sizeof(offset)), offset(0), binary(nullptr)
     {
         if (flags & HV_VLEN_DATA) {
             // variable-length binary data