NonLocalAllocator: Handle zero size allocation.

src/bgfx_p.h

@@ -2539,6 +2539,8 @@ namespace bgfx
 
 		uint64_t alloc(uint32_t _size)
 		{
+			_size = bx::min(_size, 16u);
+
 			for (FreeList::iterator it = m_free.begin(), itEnd = m_free.end(); it != itEnd; ++it)
 			{
 				if (it->m_size >= _size)