Fix "pendingBuf overflow" assert in LIT_MEM mode

[zmodem]

Since each element in s->d_buf is 2 bytes, the sx index should be multiplied by 2 in the assert.

Fixes #897

[Neustradamus]

@madler: I think that some commits are missing since 2023-11-15, can you push it? Thanks in advance.

@zmodem Issue: https://github.com/madler/zlib/issues/897

[Neustradamus]

Merged commit:

• https://github.com/madler/zlib/commit/ee474ff2d11715485a87b123edbdd615ba218b88