`switch_core_media_add_crypto()` parses the SDP `a=crypto` keysalt and
decodes it into a fixed-size key buffer. Enforce the buffer contract at
the call site:
- Reject a zero-length, negative, or over-long keysalt (the length
check now has both a lower bound and an upper bound against the copy
buffer).
- Copy the keysalt token into a NUL-terminated buffer and decode from
that, so `switch_b64_decode` stops at the token instead of reading on
into the following key material (it consumes input to the NUL and
skips non-base64 bytes).
- Pass the destination buffer size as the decode bound rather than the
parsed token length.
- Require the decoded length to cover the crypto suite's key+salt so the
subsequent copy cannot read past the decoded bytes.
Add `test_add_crypto_keysalt_bounds`, a table-driven test covering the
accepted and rejected keysalt shapes across suites (AES-128/192/256),
including RFC 4568 lifetime/MKI and multi-key lines.
