As documented in `zstd.h`, ZSTD_decompressBound returns `ZSTD_CONTENTSIZE_ERROR`
if an error occurs (not `ZSTD_CONTENTSIZE_UNKNOWN`). This is consistent with
the error checking made in ZSTD_decompressBound, particularly line 545.
Introduces a new utility function `ZSTD_findFrameCompressedSize_internal` which
is equivalent to `ZSTD_findFrameCompressSize`, but accepts an additional output
parameter `bound` that computes an upper-bound for the compressed data in the frame.
The new API function is named `ZSTD_decompressBound` to be consistent with
`zstd_compressBound` (the inverse operation). Clients will now be able to compute an upper-bound for
their compressed payloads instead of guessing a large size.
Implements https://github.com/facebook/zstd/issues/1536.
shakeelrao