Fix ResumableParser#partial_value SEGV on incomplete nested containers

[tompng]

partial_value rebuilds the open containers on a scratch copy of the value stack. Folding an empty innermost container pops nothing and pushes its result, so head climbs one past the live size; the scratch buffer was sized to exactly the live size, so the push triggered rvalue_stack_grow which reallocated the ALLOCV buffer (a non-malloc pointer) and crashed.

A parent fold always reclaims its child's slot, so head exceeds the live size by at most one -- either for the missing-value placeholder or for an empty innermost fold, never both. Size the buffer to capa + 1.

SEGV reproduction code:

parser = JSON::ResumableParser.new
parser << '{"a":{"b'
parser.parse
parser.partial_value