Fix get_int_via_sysctlbyname(..) on Jenkins

Initialize `oldlen` to the size of the value, instead of leaving the value
unitialized. Leaving it unitialized seems to work by accident on amd64 when
running 64-bit programs, but not on i386.

This matches patterns in use in other programs.

PR:		237458
Approved by:	emaste (mentor; implicit)
MFC after:	1 week
Tested on:	^/head (amd64), ^/stable/11 (i386)

lib/libc/tests/sys/sendfile_test.c

@@ -62,6 +62,8 @@ get_int_via_sysctlbyname(const char *oidname)
 	size_t oldlen;
 	int int_value;
 
+	oldlen = sizeof(int_value);
+
 	ATF_REQUIRE_EQ_MSG(sysctlbyname(oidname, &int_value, &oldlen, NULL, 0),
 	    0, "sysctlbyname(%s, ...) failed: %s", oidname, strerror(errno));
 	ATF_REQUIRE_EQ_MSG(sizeof(int_value), oldlen, "sanity check failed");