Fix nullptr-with-nonzero-offset in ucstrncmp().

     };
 
     // we're going to read a[0..15] and b[0..15] (32 bytes)
-    for ( ; a + offset + 16 <= end; offset += 16) {
+    for ( ; offset + 16 <= end - a; offset += 16) {
 #ifdef __AVX2__
         __m256i a_data = _mm256_loadu_si256(reinterpret_cast<const __m256i *>(a + offset));
         __m256i b_data = _mm256_loadu_si256(reinterpret_cast<const __m256i *>(b + offset));
@@ -938,7 +938,7 @@ static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
     }
 
     // we're going to read a[0..7] and b[0..7] (16 bytes)
-    if (a + offset + 8 <= end) {
+    if (offset + 8 <= end - a) {
         __m128i a_data = _mm_loadu_si128(reinterpret_cast<const __m128i *>(a + offset));
         __m128i b_data = _mm_loadu_si128(reinterpret_cast<const __m128i *>(b + offset));
         if (isDifferent(a_data, b_data))
@@ -948,7 +948,7 @@ static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
     }
 
     // we're going to read a[0..3] and b[0..3] (8 bytes)
-    if (a + offset + 4 <= end) {
+    if (offset + 4 <= end - a) {
         __m128i a_data = _mm_loadl_epi64(reinterpret_cast<const __m128i *>(a + offset));
         __m128i b_data = _mm_loadl_epi64(reinterpret_cast<const __m128i *>(b + offset));
         if (isDifferent(a_data, b_data))
@@ -969,7 +969,7 @@ static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
     if (l >= 8) {
         const QChar *end = a + l;
         const uint16x8_t mask = { 1, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7 };
-        while (a + 7 < end) {
+        while (7 < end - a) {
             uint16x8_t da = vld1q_u16(reinterpret_cast<const uint16_t *>(a));
             uint16x8_t db = vld1q_u16(reinterpret_cast<const uint16_t *>(b));