stb_image: UB fix in stbi__get32le

Need to do the second-part shift on uint32 not int.

stb_image.h

@@ -1674,7 +1674,8 @@ static int stbi__get16le(stbi__context *s)
 static stbi__uint32 stbi__get32le(stbi__context *s)
 {
    stbi__uint32 z = stbi__get16le(s);
-   return z + (stbi__get16le(s) << 16);
+   z += (stbi__uint32)stbi__get16le(s) << 16;
+   return z;
 }
 #endif