Fix build warnings seen on Windows

[danielpieczko]

Warnings aren't enabled in the Linux/Mac host endpoint build, but there are some from my Windows build environment (VisualStudio 2019 with nmake):

• suggestion to use memcpy_s instead of memcpy; this is ignored by the _CRT_SECURE_NO_WARNINGS define
• change an int to unsigned because of an unsigned comparison
• the write_size < 0 test is always false because write_size is unsigned; place an equivalent check before the write happens