fix some more signed shifts

stb_image.h

@@ -3544,7 +3544,7 @@ static void stbi__fill_bits(stbi__zbuf *z)
 {
    do {
       STBI_ASSERT(z->code_buffer < (1U << z->num_bits));
-      z->code_buffer |= stbi__zget8(z) << z->num_bits;
+      z->code_buffer |= (usigned int) stbi__zget8(z) << z->num_bits;
       z->num_bits += 8;
    } while (z->num_bits <= 24);
 }

stb_vorbis.c

@@ -1293,7 +1293,7 @@ static uint32 get32(vorb *f)
    x = get8(f);
    x += get8(f) << 8;
    x += get8(f) << 16;
-   x += get8(f) << 24;
+   x += (uint32) get8(f) << 24;
    return x;
 }