ImageMagick has a heap-buffer-overflow in FTXT encoder

The FTXT encoder lacks a boundary check when parsing ftxt:format, resulting in an out of bounds read.

==3040863==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x5020000085b2 at pc 0x606c1ee0c6ce bp 0x7ffee30d6150 sp 0x7ffee30d6148
READ of size 1 at 0x5020000085b2 thread T0