Use some semantically more correct buffer operations

When writing into the buffer write to free() bytes starting
at getend(), instead of buffersize_max bytes at get()
-> get() is a read pointer.

This makes no difference in practice though, as we reset
the buffer before the call, so start = end = 0.

Gbp-Dch: ignore

apt-pkg/contrib/fileutl.cc

@@ -1054,7 +1054,7 @@ public:
 	 {
 	    buffer.reset();
 	    unsigned long long actualread = 0;
-	    if (filefd->Read(buffer.get(), buffer.buffersize_max, &actualread) == false)
+	    if (filefd->Read(buffer.getend(), buffer.free(), &actualread) == false)
 	       return nullptr;
 	    buffer.bufferend = actualread;
 	    if (buffer.size() == 0)