[pixbuf] Avoid potential division by zero

Avoid potential division by zero when performing the check against
multiplication overflow.  (Note that if the width is zero then there
can be no overflow anyway, so it is then safe to bypass the check.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/pixbuf.c

 	pixbuf->len = ( width * height * sizeof ( uint32_t ) );
 
 	/* Check for multiplication overflow */
-	if ( ( ( pixbuf->len / sizeof ( uint32_t ) ) / width ) != height )
+	if ( ( width != 0 ) &&
+	     ( ( pixbuf->len / sizeof ( uint32_t ) ) / width ) != height ) {
 		goto err_overflow;
+	}
 
 	/* Allocate pixel data buffer */
 	pixbuf->data = umalloc ( pixbuf->len );