Attempting to read 1d9999999999999 exhausts the heap

Attempting to read 1d9999999999999 causes ECL to hang in a busy loop and allocate memory until it runs out of heap space.

     VERSION "21.2.1"
      VCS-ID "c646799145538997d84ed6d8755be7e7837eb7ef"
          OS "Linux"
  OS-VERSION "5.15.0-2-amd64"
MACHINE-TYPE "x86_64"
    FEATURES (:WALKER :CDR-1 :CDR-5 :LINUX :FORMATTER :CDR-7 :ECL-WEAK-HASH
              :LITTLE-ENDIAN :ECL-READ-WRITE-LOCK :LONG-LONG :UINT64-T
              :UINT32-T :UINT16-T :COMPLEX-FLOAT :LONG-FLOAT :UNICODE :DFFI
              :CLOS-STREAMS :CMU-FORMAT :UNIX :ECL-PDE :DLOPEN :CLOS :THREADS
              :BOEHM-GC :ANSI-CL :COMMON-LISP :FLOATING-POINT-EXCEPTIONS
              :IEEE-FLOATING-POINT :PACKAGE-LOCAL-NICKNAMES :CDR-14
              :PREFIXED-API :FFI :X86_64 :COMMON :ECL)