msm: vidc: Add check to avoid NULL ptr dereference

v4l2_ctrl_get_name may return NULL that leads to NULL
ptr dereference. So added necessay check to avoid ptr
dereferencing if it is NULL.

Change-Id: If1a4ee8fe22e240b278f31888637d22b81b334e1
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>