util/u_format: Assert that format block size is at least 1 byte.

The block size for all formats is currently at least 1 byte. Add an
assertion for this.

This should silence several Coverity "Division or modulo by zero"
defects.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/util/u_format.h

@@ -716,10 +716,15 @@ static INLINE uint
 util_format_get_blocksize(enum pipe_format format)
 {
    uint bits = util_format_get_blocksizebits(format);
+   uint bytes = bits / 8;
 
    assert(bits % 8 == 0);
+   assert(bytes > 0);
+   if (bytes == 0) {
+      bytes = 1;
+   }
 
-   return bits / 8;
+   return bytes;
 }
 
 static INLINE uint