netutils/webclient: add NULL pointer check after alloc buffer
netutils/webclient: add NULL pointer check after alloc buffer

This commit addes NULL pointer check routine in webclient after
allocating buffer.

Change-Id: I3e9f2fd6606d72f0b92f3f80a274d095a4b5b2c2
Signed-off-by: Junyeon LEE <ddc45265127aa1feb4b01be10ec1f0d25d062d3d@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT