stb_image: avoid signed left shifts

stb_image.h

@@ -1767,7 +1767,7 @@ static int stbi__build_huffman(stbi__huffman *h, int *count)
       if (h->size[k] == j) {
          while (h->size[k] == j)
             h->code[k++] = (stbi__uint16) (code++);
-         if (code-1 >= (1 << j)) return stbi__err("bad code lengths","Corrupt JPEG");
+         if (code-1 >= (1u << j)) return stbi__err("bad code lengths","Corrupt JPEG");
       }
       // compute largest code + 1 for this size, preshifted as needed later
       h->maxcode[j] = code << (16-j);
@@ -1811,7 +1811,7 @@ static void stbi__build_fast_ac(stbi__int16 *fast_ac, stbi__huffman *h)
             if (k < m) k += (~0U << magbits) + 1;
             // if the result is small enough, we can fit it in fast_ac table
             if (k >= -128 && k <= 127)
-               fast_ac[i] = (stbi__int16) ((k << 8) + (run << 4) + (len + magbits));
+               fast_ac[i] = (stbi__int16) ((k * 256) + (run * 16) + (len + magbits));
          }
       }
    }
@@ -1820,7 +1820,7 @@ static void stbi__build_fast_ac(stbi__int16 *fast_ac, stbi__huffman *h)
 static void stbi__grow_buffer_unsafe(stbi__jpeg *j)
 {
    do {
-      int b = j->nomore ? 0 : stbi__get8(j->s);
+      unsigned int b = j->nomore ? 0 : stbi__get8(j->s);
       if (b == 0xff) {
          int c = stbi__get8(j->s);
          while (c == 0xff) c = stbi__get8(j->s); // consume fill bytes