[YAMLIO] Support non-null-terminated inputs

In some places the parser guards against dereferencing `End`, while in
others it relies on the presence of a trailing `'\0'` to elide checks.

Add the remaining guards needed to ensure the parser never attempts to
dereference `End`, making it safe to not require a null-terminated input
buffer.

Update the parser fuzzer harness so that it tests with buffers that are
guaranteed to be non-null-terminated, null-terminated, and 1-terminated,
additionally ensuring the result of the parse is the same in each case.

Some of the regression tests were written by inspection, and some are
cases caught by the fuzzer which required additional fixes in the
parser.

Differential Revision: https://reviews.llvm.org/D84050