Both pr -0' and e.g., pr -03' would evoke `column count too large'.

`pr -0' should give a better diagnostic and `pr -03' should be
equivalent to `pr -3'.

(parse_column_count): Change return type to void.
Call error (EXIT_FAILURE, ... for an invalid string.
(main): Allocate space for column_count_string using malloc.
Accumulate all old-style column-count digits before converting.
When the number of columns is specified via both old-style,
(e.g., -3), and a long option (--columns=5), ensure that only
the last one specified takes effect.

Add tests for the above.

tests/pr/Test.pm

@@ -346,6 +346,15 @@ my @tv = (
 # This test would fail with textutils-2.1 and earlier.
 ['col-last', '-W3 -t2', "a\nb\nc\n", "a c\nb\n", 0],
 
+# Make sure that -02 is treated just like -2.
+['col-02', '-W3 -t -02',                "a\nb\nc\n", "a c\nb\n", 0],
+# The -2 must override preceding column-count-specifying options.
+['col-2', '-W3 -t -4 --columns=1 -2', "a\nb\nc\n", "a c\nb\n", 0],
+# The --columns=2 must override preceding column-count-specifying options.
+['col-long', '-W3 -t -1 --columns=2',     "a\nb\nc\n", "a c\nb\n", 0],
+# Make sure these fail.
+['col-0', '-0', '', '', 1],
+['col-inval', '-'.'9'x100, '', '', 1],
 );
 #']]);
 
@@ -364,6 +373,7 @@ sub test_vector
       $flags = "$common_option_prefix$sep$flags";
       push (@new_tv, [$test_name, $flags, $in, $exp, $ret]);
 
+      # For any use of -N, create an identical test with --columns=N.
       (my $new_flags = $flags) =~ s/(^| )-(\d+)( |$)/$1--columns=$2$3/g;
       $new_flags ne $flags
 	and push (@new_tv, ["$test_name.C", $new_flags, $in, $exp, $ret]);