Fix md_get_all() return value

[asvetlov]

It is a follow-up PR for #1187 that corrects retval of internal md_get_all() function.

[asvetlov]

Thanks @Vizonex for catching.

The PR doesn't need a changenote because it corrects an internal function, and #1187 has not been released yet.

Actually, the retval is not used yet but the work is a preparation for #1183

[codspeed-hq]

CodSpeed Performance Report

Merging #1194 will degrade performances by 32.5%

_{Comparing asvetlov:fix-getall-retval (764b3a4) with master (eeb048f)}

Summary

❌ 2 regressions
✅ 242 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_cimultidict_getall_istr_miss[py]	246.6 µs	362 µs	-31.88%
❌	test_multidict_getall_str_miss[ci-py]	244.1 µs	361.6 µs	-32.5%