common: cast arguments to character classification functions to unsigned char

common: cast arguments to character classification functions to unsigned char

We get failures of this type on the cygwin CI machine:

    15:28:20 common.c: In function `bt_common_string_is_printable`:
    15:28:20 common.c:786:16: error: array subscript has type `char` [-Werror=char-subscripts]
    15:28:20   786 |   if (!isprint(*ch) && *ch != '\n' && *ch != '\r' &&
    15:28:20       |                ^~~

This error only pops up on some platforms that have isprint implemented
using a lookup table.  This table is indexed using `*ch`, which is a
char.  And because char is signed on some platforms, gcc warns that this
is dangerous: we could access the array with a negative index, which
would yield unexpected results.

This is on purpose in newlib (the libc used by cygwin, apparently), see
this comment:

  https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libc/include/ctype.h;h=a0009af17485acc3d70586a0051269a7a9c350d5;hb=HEAD#l78

The Linux man page for isprint also mentions it:

       The standards require that the argument c for these functions is
       either EOF or a value that is representable in the type unsigned
       char.  If the argument c is of type char, it must be cast to unsigned
       char, as in the following example:

           char c;
           ...
           res = toupper((unsigned char) c);

       This is necessary because char may be the equivalent of signed char,
       in which case a byte where the top bit is set would be sign extended
       when converting to int, yielding a value that is outside the range of
       unsigned char.

Add casts to unsigned char to fix the various instances of this error.

Change-Id: Ice2305490997f595c6f5140a8be2abaa7fd1d8f6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3194
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>