basenc: --base16: support lower case hex digits

* src/basenc.c (base16_decode_ctx): Convert to uppercase
before converting from hex.
* tests/basenc/basenc.pl: Add a test case.
* NEWS: Mention the change in behavior.
Addresses https://bugs.gnu.org/66698

NEWS

@@ -19,6 +19,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   base32 and base64 no longer require padding when decoding.
   Previously an error was given for non padded encoded data.
 
+  basenc --base16 -d now supports lower case hexadecimal characters.
+  Previously an error was given for lower case hex digits.
+
   ls --dired now implies long format output without hyperlinks enabled,
   and will take precedence over previously specified formats or hyperlink mode.
 

src/basenc.c

@@ -577,7 +577,7 @@ base16_decode_ctx (struct base_decode_context *ctx,
           continue;
         }
 
-      int nib = *in++;
+      int nib = c_toupper (*in++);
       if ('0' <= nib && nib <= '9')
         nib -= '0';
       else if ('A' <= nib && nib <= 'F')

tests/basenc/basenc.pl

@@ -159,6 +159,7 @@ my @Tests =
  ['b16_7', '--base16 -d',     {IN=>'G'}, {EXIT=>1},
   {ERR=>"$prog: invalid input\n"}],
  ['b16_8', '--base16 -d',     {IN=>"AB\nCD"}, {OUT=>"\xAB\xCD"}],
+ ['b16_9', '--base16 -d',     {IN=>lc ($base16_out)},  {OUT=>$base16_in}],