[lib] Fix UBSAN warning in ZSTD_decompressSequences()

[terrelln]

This moves the pointer addition below the error check, and also moves BIT_reloadDStream() to the bottom of the loop.

Benchmarks run at level 1 on the silesia corpus.

Platform	Compiler	dev	PR
i9-9900k	gcc 10.2	1213 MB/s	1207 MB/s
i9-9900k	clang 11.1	1032 MB/s	1072 MB/s
Macbook	clang	1269 MB/s	1271 MB/s
devserver	gcc	840 MB/s	846 MB/s
devserver	clang	761 MB/s	780 MB/s

Fixes #2590.