[BUG] assignments preceding 'read' persist past command in POSIX mode

[Martijn Dekker]

In bash-20180511 and bash-20180518 snapshots and in bash 5.0.0-alpha, in 
POSIX mode, assignments preceding 'read' incorrectly persist past the 
command. 'read' is a regular builtin so they should not persist.

$ ./bash -o posix -c 'v=ok; v=bug read x </dev/null; echo "$v"'
bug

This is especially problematic because IFS assignments persist too.

Thanks,

- M.