add a redundant loop condition to silence GCC warnings

See:
#94
#183
#189

c/blake3.c

@@ -340,12 +340,18 @@ INLINE void compress_subtree_to_parent_node(
   uint8_t cv_array[MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN];
   size_t num_cvs = blake3_compress_subtree_wide(input, input_len, key,
                                                 chunk_counter, flags, cv_array);
+  assert(num_cvs <= MAX_SIMD_DEGREE_OR_2);
 
   // If MAX_SIMD_DEGREE is greater than 2 and there's enough input,
   // compress_subtree_wide() returns more than 2 chaining values. Condense
   // them into 2 by forming parent nodes repeatedly.
   uint8_t out_array[MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN / 2];
-  while (num_cvs > 2) {
+  // The second half of this loop condition is always true, and we just
+  // asserted it above. But GCC can't tell that it's always true, and if NDEBUG
+  // is set on platforms where MAX_SIMD_DEGREE_OR_2 == 2, GCC emits spurious
+  // warnings here. GCC 8.5 is particular sensitive, so if you're changing this
+  // code, test it against that version.
+  while (num_cvs > 2 && num_cvs <= MAX_SIMD_DEGREE_OR_2) {
     num_cvs =
         compress_parents_parallel(cv_array, num_cvs, key, flags, out_array);
     memcpy(cv_array, out_array, num_cvs * BLAKE3_OUT_LEN);