[BUG] unsigned len is set to -1

[photoszzt]

Describe the bug

ava/common/endpoint_lib.cpp

Lines 259 to 261 in 2bf00b3

	size_t len = -1;
	void *data = metadata->extract(obj, &len);
	assert(len != -1);

Comparing signed and unsigned integers. Here len is unsigned but its set to -1 which causes an overflow.