[zstd][leak] Avoid memory leak on early return of ZSTD_generateSequence

[zstd][leak] Avoid memory leak on early return of ZSTD_generateSequence

Sanity checks on a few of the context parameters (i.e. workers and block size)
may prompt an early return on ZSTD_generateSequences.

Allocating the destination buffer past those return points avoids a potential
memory leak.

This patch should fix issue #4112.