pngread: avoid clang warning for unreachable code

```
pngread.c:2841:10: warning: 'break' will never be executed [-Wunreachable-code-break]
         break;
         ^~~~~
```

pngread.c

@@ -2838,7 +2838,6 @@ png_image_read_colormap(png_voidp argument)
       default:
          png_error(png_ptr, "invalid PNG color type");
          /*NOT REACHED*/
-         break;
    }
 
    /* Now deal with the output processing */