ImageMagick: MSL attribute stack buffer overflow leads to out of bounds write.

A stack buffer overflow occurs when processing the an attribute in msl.c. A long value overflows a fixed-size stack buffer, leading to memory corruption.

=================================================================
==278522==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffdb8c76984 at pc 0x55a4bf16f507 bp 0x7ffdb8c75bc0 sp 0x7ffdb8c75bb0
WRITE of size 1 at 0x7ffdb8c76984 thread T0