Change `read_string` to fail when not the entire input has been
consumed. This avoids unexpected, even dangerous behavior (fixes#6223).
The new JSON parser adapted in #6121 also solves this problem so in
master this is a temporary fix, but should be backported to older releases.
Also adds tests for the new behavior.
Github-Pull: #6226
Rebased-From: 4e157fc60dae5ca69933ea4c1585a2a078b4d957