hush: fix bkslash+newline handling and number validation in ${NN} and ${#NN}

Entering "${1a}" into interactive shell was making it exit.

function                                             old     new   delta
parse_dollar                                         824     958    +134
i_getch_and_eat_bkslash_nl                             -      44     +44
parse_expr                                           917     938     +21
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(add/remove: 1/0 grow/shrink: 2/0 up/down: 199/0)             Total: 199 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
6 files changed