Fix #29; Fix incorrect offset calculation

To calculate "offset" to store a new plan at, we should sum the lengths
of the already stored plans within the memory plus the number of plans
to accomodate for NULL character at the end of each plan.

Instead, I used to sum up the lengths of all plans and add 1.

Author: kovmir

pg_show_plans.c

@@ -292,9 +292,7 @@ append_query_plan(ExplainState *es)
 
 	offset = 0;
 	for (i = 0; i < nest_level; i++)
-		offset += pgsp_cache->plan_len[i];
-	if (nest_level >= 1)
-		offset++; /* Point right after the '\0'. */
+		offset += pgsp_cache->plan_len[i] + 1;
 	space_left = max_plan_length - offset;
 
 	if (pgsp->plan_format == EXPLAIN_FORMAT_TEXT)
@@ -637,9 +635,7 @@ pg_show_plans(PG_FUNCTION_ARGS)
 		 * count offset to get the desired plan. */
 		offset = 0;
 		for (i = 0; i < curr_nest; i++)
-			offset += pgvp_tmp_entry->plan_len[i];
-		if (offset > 0)
-			offset++;
+			offset += pgvp_tmp_entry->plan_len[i] + 1;
 
 		MemSet(nulls, 0, sizeof(nulls));
 		values[0] = Int32GetDatum(pgvp_tmp_entry->hash_key.pid);