tests: Check lseek, read and malloc results with correct types in test.

tests: Check lseek, read and malloc results with correct types in test.

When compiling dwfl-report-offline-memory.c on some systems (latest
gcc/glibc and --enable-sanitize-undefined) we might get:

In file included from /usr/include/features.h:490,
                 from /usr/include/assert.h:35,
                 from dwfl-report-offline-memory.c:18:
In function ‘read’,
    inlined from ‘main’ at dwfl-report-offline-memory.c:68:23:
/usr/include/bits/unistd.h:38:10: error: ‘__read_alias’ specified size 18446744073709551615
  exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]
   38 |   return __glibc_fortify (read, __nbytes, sizeof (char),
      |          ^~~~~~~~~~~~~~~
/usr/include/bits/unistd.h: In function ‘main’:
/usr/include/bits/unistd.h:26:16: note: in a call to function ‘__read_alias’ declared with
  attribute ‘access (write_only, 2, 3)’
   26 | extern ssize_t __REDIRECT (__read_alias, (int __fd, void *__buf,
      |                ^~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [Makefile:2461: dwfl-report-offline-memory.o] Error 1

Fix by using the correct types and checking all return values.

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