libelf: Set dst to zero on failure in __elf_getphdrnum_rdlock

libelf: Set dst to zero on failure in __elf_getphdrnum_rdlock

GCC with -flto finds some (very) unlikely error paths. It flags callers
of __elf_getphdrnum_chk_rdlock with *dst not yet set because an internal
call to __elf_getphdrnum_rdlock might not initialize *dst either in one
particular failure path. The sanity check that __elf_getphdrnum_chk_rdlock
then performs might happen against uninitialized data. So just set *dst
to zero on failure in __elf_getphdrnum_rdlock so any caller can simply
check either the result/error code or whether *dst is zero or not.

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