[runtime] Fixed flags for VirtualAlloc in mono_valloc (Windows)

If ```VirtualAlloc``` gets non-null address and MEM_COMMIT flag (without MEM_RESERVE), it assumes you want to commit memory which is already reserved. What we want here is reserve AND commit the memory.

This fixes random crashes occuring on Windows after commit https://github.com/mono/mono/commit/2fed14d6ef5e0f0f5fe71f0f5a18a1146cb1113d. Before this commit ```mono_valloc``` was not used with ```addr``` argument (at least on Windows).