Assert on first thread allocation on 3.5.2

Hey.
Just updated to 3.5.2 and it asserts about missing theap in the very first allocation on new thread.

Editor_debug.exe!_mi_assert_fail(const char * assertion, const char * fname, unsigned int line, const char * func) Line 572 C++
Editor_debug.exe!mi_theap_malloc(mi_theap_s * theap, unsigned __int64 size) Line 278 C++
Editor_debug.exe!mi_theap_alloc_new(mi_theap_s * theap, unsigned __int64 size) Line 830 C++
Editor_debug.exe!mi_new(unsigned __int64 size) Line 837 C++
Editor_debug.exe!operator new(unsigned __int64 n) Line 40 C++

mi_new() -> _mi_theap_default() seems to return nullptr. 100% occurrence. 3.5.1 verified to behave correct.

Start with the reported stack through mi_new, _mi_theap_default(), mi_theap_alloc_new(), and mi_theap_malloc(). Reproduce the first allocation on a new thread with 3.5.2, compare against 3.5.1, and trace why _mi_theap_default() returns nullptr; done means the assertion no longer occurs in the reported scenario.

Written by the indexing model from the issue text.