Release connection when query cannot be built

This was leaking connection when a query could not be build - ie if an invalid column name was used.
More info here - https://github.com/crystal-lang/crystal-mysql/issues/40

src/db/pool_prepared_statement.cr

@@ -34,7 +34,12 @@ module DB
       clean_connections
       conn, existing = @db.checkout_some(@connections)
       @connections << WeakRef.new(conn) unless existing
-      conn.prepared.build(@query)
+      begin
+        conn.prepared.build(@query)
+      rescue ex
+        conn.release
+        raise ex
+      end
     end
 
     private def clean_connections