sad_test: initialize bit_depth_ in all cases

previously 'bit_depth_', which is later used to calculate 'mask_', would
be left uninitialized in non-high-bitdepth builds

Change-Id: Ia72035f4645baf3bb0f191504f491b934cdf1e0e

test/sad_test.cc

@@ -140,6 +140,8 @@ class SADTestBase : public ::testing::Test {
       reference_data_ = CONVERT_TO_BYTEPTR(reference_data16_);
       second_pred_ = CONVERT_TO_BYTEPTR(second_pred16_);
     }
+#else
+    bit_depth_ = VPX_BITS_8;
 #endif
     mask_ = (1 << bit_depth_) - 1;
     source_stride_ = (width_ + 31) & ~31;