decNumberToString calls for a buffer that can hold a string of digits+14
characters, not a buffer of size digits+14.
We need to allocate an extra byte for the NUL byte.
-10E-1000010001, for example, will be stringified as -1.0E-1000010000
and decNumberToString will currently write an extra NUL byte after the
allocated buffer in the heap.
Originally reported by @SEU-SSL on GitHub.
Ref: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64574
Fixes GHSA-686w-5m7m-54vc
Emanuele Torre
71c2ab509a