Compiler warning on Windows (signed/unsigned mismatch)

[janogran]

When compiling blake3 for Windows VS2019, there is a compiler warning caused be the lines below from blake3.c:

// Recurse!
size_t left_n = -1;
size_t right_n = -1;

blake3.c
C:__w\2827\s\Base\src\blake3\blake3.c(306,1): error C2220: the following warning is treated as an error [C:__w\2827\s\work\x64_release\Base\src\Base.vcxproj]
C:__w\2827\s\Base\src\blake3\blake3.c(306,1): warning C4245: 'initializing': conversion from 'int' to 'size_t', signed/unsigned mismatch [C:__w\2827\s\work\x64_release\Base\src\Base.vcxproj]
C:__w\2827\s\Base\src\blake3\blake3.c(307,1): warning C4245: 'initializing': conversion from 'int' to 'size_t', signed/unsigned mismatch [C:__w\2827\s\work\x64_release\Base\src\Base.vcxproj]

If warnings are treated as errors (as they are on my system), the build fails due to this.

Preferred solution:
Initiate left_n and right_n to SIZE_MAX, not -1 (or explicitly cast to size_t)