libdwfl: Rewrite reading of ar_size in elf_begin_rand

libdwfl: Rewrite reading of ar_size in elf_begin_rand

With GCC 12.1.1, glibc 2.35, -fsanitize=undefined and
-D_FORTIFY_SOURCE=3 we get the following error message:

In file included from /usr/include/ar.h:22,
                 from ../libelf/libelfP.h:33,
                 from core-file.c:31:
In function ‘pread’,
    inlined from ‘pread_retry’ at ../lib/system.h:188:21,
    inlined from ‘elf_begin_rand’ at core-file.c:86:16,
    inlined from ‘core_file_read_eagerly’ at core-file.c:205:15:
/usr/include/bits/unistd.h:74:10: error: ‘__pread_alias’ writing 58 or more bytes into a region of size 10 overflows the destination [-Werror=stringop-overflow=]
   74 |   return __glibc_fortify (pread, __nbytes, sizeof (char),
      |          ^~~~~~~~~~~~~~~
/usr/include/ar.h: In function ‘core_file_read_eagerly’:
/usr/include/ar.h:41:10: note: destination object ‘ar_size’ of size 10
   41 |     char ar_size[10];           /* File size, in ASCII decimal.  */
      |          ^~~~~~~
/usr/include/bits/unistd.h:50:16: note: in a call to function ‘__pread_alias’ declared with attribute ‘access (write_only, 2, 3)’
   50 | extern ssize_t __REDIRECT (__pread_alias,
      |                ^~~~~~~~~~
cc1: all warnings being treated as errors

The warning disappears when dropping either -fsanitize=undefined
or when using -D_FORTIFY_SOURCE=2. It looks like a false positive.
But I haven't figured out how/why it happens.

The code is a little tricky to proof correct though. The ar_size
field is a not-zero terminated string ASCII decimal, right-padded
with spaces. Which is then converted with strtoll. Relying on the
fact that the struct ar_hdr is zero initialized, so there will be
a zero byte after the ar_size field.

Rewrite the code to just use a zero byte terminated char array.
Which is much easier to reason about. As a bonus the error disappears.

Signed-off-by: Mark Wielaard <mark@klomp.org>