hush: handle ${var:NUM:} too

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

shell/hush_test/hush-vars/param_expand_bash_substring.right

@@ -7,23 +7,35 @@ hush: syntax error: unterminated ${name}
 1:1  =||
 1:1:2=||
 1::2 =||
+1:1: =||
+1::  =||
 1    =|0123|
 1:1  =|123|
 1:1:2=|12|
 1::2 =|01|
+1:1: =||
+1::  =||
 f    =||
 f:1  =||
 f:1:2=||
 f::2 =||
+f:1: =||
+f::  =||
 f    =||
 f:1  =||
 f:1:2=||
 f::2 =||
+f:1: =||
+f::  =||
 f    =|a|
 f:1  =||
 f:1:2=||
 f::2 =|a|
+f:1: =||
+f::  =||
 f    =|0123456789|
 f:1  =|123456789|
 f:1:2=|12|
 f::2 =|01|
+f:1: =||
+f::  =||