libdwfl: Don't allocate more than SIZE_MAX in dwfl_segment_report_module.

libdwfl: Don't allocate more than SIZE_MAX in dwfl_segment_report_module.

The code in dwfl_segment_report_module tries to allocate and fill in
memory as described in a core file. Normally all memory in filled in
through the (phdrs) memory_callback or the read_eagerly callback. If
the last callback doesn't work we try to calloc file_trimmed_end bytes
and then try to fill in the parts of memory we can from the core file
at the correct offsets.

file_trimmed_end is a GElf_Off which is an unsigned 64bit type. On
32bit systems this means when cast to a size_t to do an allocation
might allocate truncated (much smaller) value. So make sure to not
allocate more than SIZE_MAX bytes.

It would be nice to have a better way to limit the amount of memory
allocated here. A core file might describe really big memory areas for
which it doesn't provide any data. In that case we really shouldn't
calloc mega- or giga-bytes of zeroed out memory.

Reported-by: Evgeny Vereshchagin <evvers@ya.ru>
Signed-off-by: Mark Wielaard <mark@klomp.org>