Med: crm_get_msec: prevent integer overflow + test

Before (sizeof(long long) == 8):
input: 9223372036854775807s, output: -1000 (expected 9223372036854775807)
[...]

After:
input: 9223372036854775807s, output: 9223372036854775807
[...]

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>