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
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/0 up/down: 199/0)             Total: 199 bytes

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

shell/ash_test/ash-parsing/bkslash_newline4.right

@@ -0,0 +1,4 @@
+1:1
+22:22
+3:3
+Ok:0

shell/ash_test/ash-parsing/bkslash_newline4.tests

@@ -0,0 +1,14 @@
+set -- 1 22 333
+echo 1:$\
+1
+echo 22:$\
+{\
+2\
+}
+echo 3:$\
+{\
+#\
+3\
+}
+echo Ok:$\
+?