Fix a pair of warnings in stb_image.h

stb_image.h

@@ -96,7 +96,7 @@ RECENT REVISION HISTORY:
     Ryamond Barbiero        Paul Du Bois       Engin Manap        github:snagar
     Michaelangel007@github  Oriol Ferrer Mesia Dale Weiler        github:Zelex
     Philipp Wiesemann       Josh Tobin         github:rlyeh       github:grim210@github
-    Blazej Dariusz Roszkowski                  github:sammyhw
+    Blazej Dariusz Roszkowski                  github:sammyhw     GrayHatter@uTox
 
 */
 
@@ -1761,7 +1761,7 @@ static void stbi__build_fast_ac(stbi__int16 *fast_ac, stbi__huffman *h)
             // magnitude code followed by receive_extend code
             int k = ((i << len) & ((1 << FAST_BITS) - 1)) >> (FAST_BITS - magbits);
             int m = 1 << (magbits - 1);
-            if (k < m) k += (-1 << magbits) + 1;
+            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));
@@ -3594,6 +3594,7 @@ static stbi_uc *load_jpeg_image(stbi__jpeg *z, int *out_x, int *out_y, int *comp
 
 static void *stbi__jpeg_load(stbi__context *s, int *x, int *y, int *comp, int req_comp, stbi__result_info *ri)
 {
+   (void)ri; // This unused warning is safe to ignore.
    unsigned char* result;
    stbi__jpeg* j = (stbi__jpeg*) stbi__malloc(sizeof(stbi__jpeg));
    j->s = s;