Fix a build warning
Fix a build warning

This patch fixed the following build warning.
In function ‘row_mt_worker_hook’:
warning: variable ‘num_planes’ might be clobbered by ‘longjmp’ or
‘vfork’.

Change-Id: I9ff2a712eda389d7d92a677d2a6164197f238a9e

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom