util/ForeignFifoBuffer: shift the buffer in MoveFrom() on demand

The API documentation says "move as much data as possible", and if
there is room at the head of the buffer, we should use that if the
room after the tail is not large enough.

src/util/ForeignFifoBuffer.hxx

@@ -223,6 +223,15 @@ public:
 	size_type MoveFrom(ForeignFifoBuffer<T> &src) noexcept {
 		auto r = src.Read();
 		auto w = Write();
+
+		if (w.size < r.size && head > 0) {
+			/* if the source contains more data than we
+			   can append at the tail, try to make more
+			   room by shifting the head to 0 */
+			Shift();
+			w = Write();
+		}
+
 		size_t n = std::min(r.size, w.size);
 
 		std::move(r.data, r.data + n, w.data);