remove cases of equal comparison for qsort()

Returning 0 (equal) can lead to undefined behaviour.
And, in our cases we'll never have equal keys (added asserts for that)

Change-Id: Ifaf202df321d3f877ad2a03de42e0d6cdd1b2388

src/enc/vp8l.c

@@ -37,7 +37,8 @@ extern "C" {
 static int CompareColors(const void* p1, const void* p2) {
   const uint32_t a = *(const uint32_t*)p1;
   const uint32_t b = *(const uint32_t*)p2;
-  return (a < b) ? -1 : (a > b) ? 1 : 0;
+  assert(a != b);
+  return (a < b) ? -1 : 1;
 }
 
 // If number of colors in the image is less than or equal to MAX_PALETTE_SIZE,

src/utils/huffman_encode.c

@@ -138,13 +138,8 @@ static int CompareHuffmanTrees(const void* ptr1, const void* ptr2) {
   } else if (t1->total_count_ < t2->total_count_) {
     return 1;
   } else {
-    if (t1->value_ < t2->value_) {
-      return -1;
-    }
-    if (t1->value_ > t2->value_) {
-      return 1;
-    }
-    return 0;
+    assert(t1->value_ != t2->value_);
+    return (t1->value_ < t2->value_) ? -1 : 1;
   }
 }