fix bug in png decoding with 1,2,4-bpp images using filter that samples previous line

stb_image.h

@@ -4201,7 +4201,7 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
 
    for (j=0; j < y; ++j) {
       stbi_uc *cur = a->out + stride*j;
-      stbi_uc *prior = cur - stride;
+      stbi_uc *prior;
       int filter = *raw++;
 
       if (filter > 4)
@@ -4213,6 +4213,7 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
          filter_bytes = 1;
          width = img_width_bytes;
       }
+      prior = cur - stride; // bugfix: need to compute this after 'cur +=' computation above
 
       // if first row, use special filter that doesn't sample previous row
       if (j == 0) filter = first_row_filter[filter];