Fix leb128 reading

Fix leb128 reading

PR 26773 points out that some sleb128 values are decoded incorrectly.

This version of the fix only examines the sleb128 conversion.
Overlong encodings are not handled, and the uleb128 decoders are not
touched.  The approach taken here is to do the work in an unsigned
type, and then rely on an implementation-defined cast to convert to
signed.

Signed-off-by: Tom Tromey <tom@tromey.com>