Fix build warnings from use of & to create a pointer to buffer.

[kbongort]

Use withUnsafeBytes() instead, as recommended.

The warnings (which occur only on Linux, iOS, and tvOs) are:

/root/.build/checkouts/swifter/XCode/Sources/Socket+File.swift:24:52: warning: inout expression creates a temporary pointer, but argument #1 should be a pointer that outlives the call to '+'
                    let writeResult = send(target, &buffer + writeCounter, readResult - writeCounter, Int32(MSG_NOSIGNAL))
                                                   ^~~~~~~
/root/.build/checkouts/swifter/XCode/Sources/Socket+File.swift:24:52: note: implicit argument conversion from '[UInt8]' to 'UnsafeRawPointer' produces a pointer valid only for the duration of the call to '+'
                    let writeResult = send(target, &buffer + writeCounter, readResult - writeCounter, Int32(MSG_NOSIGNAL))
                                                   ^~~~~~~
/root/.build/checkouts/swifter/XCode/Sources/Socket+File.swift:24:52: note: use the 'withUnsafeBytes' method on Array in order to explicitly convert argument to buffer pointer valid for a defined scope
                    let writeResult = send(target, &buffer + writeCounter, readResult - writeCounter, Int32(MSG_NOSIGNAL))

[swifter-bot]

	1 Message
📖	Hey, @kbongort 👋.

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[Vkt0r]

Thanks for taking care of this 👍. This looks good.