stb_image: Avoid shift of signed values in extend_receive

Use an equivalent formulation that has sgn=0 or 1, not 0 or -1.
This avoids right-shifting signed values, at least in this place.

Fixes issue #1061.

stb_image.h

@@ -2102,12 +2102,12 @@ stbi_inline static int stbi__extend_receive(stbi__jpeg *j, int n)
    int sgn;
    if (j->code_bits < n) stbi__grow_buffer_unsafe(j);
 
-   sgn = (stbi__int32)j->code_buffer >> 31; // sign bit is always in MSB
+   sgn = j->code_buffer >> 31; // sign bit always in MSB; 0 if MSB clear (positive), 1 if MSB set (negative)
    k = stbi_lrot(j->code_buffer, n);
    j->code_buffer = k & ~stbi__bmask[n];
    k &= stbi__bmask[n];
    j->code_bits -= n;
-   return k + (stbi__jbias[n] & ~sgn);
+   return k + (stbi__jbias[n] & (sgn - 1));
 }
 
 // get some unsigned bits