fix valid_number_of_base

function execution halts entirely on a invalid base, so we never
actually get to our 'return 1'. I'm not sure how our expected failures
ever actually worked, but it's quite plain to see that:

  $ a=$(( 2#3 )); echo hi

Never echos "hi". Use a subshell in valid_number_of_base so that we can
properly "trap" execution, and return a meaningful result.

common

@@ -95,11 +95,15 @@ chroot_umount() {
   umount "${CHROOT_ACTIVE_MOUNTS[@]}"
 }
 
+try_cast() (
+  _=$(( $1#$2 ))
+) 2>/dev/null
+
 valid_number_of_base() {
   local base=$1 len=${#2} i=
 
   for (( i = 0; i < len; i++ )); do
-    { _=$(( $base#${2:i:1} )) || return 1; } 2>/dev/null
+    try_cast "$base" "${2:i:1}" || return 1
   done
 
   return 0

test/test_valid_number_of_base

@@ -11,4 +11,5 @@ EXPECT_success valid_number_of_base 10 1234567890
 EXPECT_failure valid_number_of_base 10 1234.567890
 EXPECT_failure valid_number_of_base 10 1234567890abcdef
 
-EXPECT_success valid_number_of_base 18 1234567890abcdefghi
+EXPECT_success valid_number_of_base 18 1234567890abcdefgh
+EXPECT_failure valid_number_of_base 18 1234567890abcdefghi