add missing ()s to macro parameters

BUG=webp:355

Change-Id: I616c6d3540d6551edd1b1cfdb5bffcf0a044c90f

src/dsp/filters.c

@@ -20,8 +20,8 @@
 // Helpful macro.
 
 # define SANITY_CHECK(in, out)                                                 \
-  assert(in != NULL);                                                          \
-  assert(out != NULL);                                                         \
+  assert((in) != NULL);                                                        \
+  assert((out) != NULL);                                                       \
   assert(width > 0);                                                           \
   assert(height > 0);                                                          \
   assert(stride >= width);                                                     \