src2 is used in CAVS_SUBPIX_HV iff FULL is true (it is exactly
for the egpr functions); otherwise it might be NULL. So check
for FULL before doing pointer arithmetic.
Fixes a "src/libavcodec/cavsdsp.c:524:1: runtime error: applying
non-zero offset 8 to null pointer" from UBSan.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>