VT: fixed "Uninitialized scalar field" reported by static analysis

[Problem] Uninitialized scalar field (UNINIT_CTOR) for
          member variables of RTPSource & ARTPWriter
[Solution] initialize variables in constructor.

Bug: 175266635

Change-Id: I5b41cbe2090803a40852102afbe97e34cde79895
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>