camera: fix vendortags section malloc failure

size_t type variables should be initialized properly especially on 64-bit archs
if they are type casted to shorter datatypes for some operation and then later
used in entirety. The results may be unpredictable since the most significant
bytes may contain garbage.

Bug 18222311

Change-Id: Iced82e964cf8c98f6856d257fc1534a5237a9064