ImageMagick has heap-based buffer overflow in UHDR encoder

A heap-based buffer overflow in the UHDR encoder can happen due to truncation of a value and it would allow an out of bounds write.

================================================================
==2158399==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x521000039500 at pc 0x562a4a42f968 bp 0x7ffcca4ed6c0 sp 0x7ffcca4ed6b0
WRITE of size 1 at 0x521000039500 thread T0