Session: Fix mismatched delete operator.

Valgrind reported this here:

Mismatched free() / delete / delete []
   at 0x4C2D31B: operator delete(void*) (vg_replace_malloc.c:576)
   by 0x1C76D0: operator() (unique_ptr.h:78)
   by 0x1C76D0: ~unique_ptr (unique_ptr.h:268)
   by 0x1C76D0: pv::data::AnalogSegment::append_interleaved_samples(float const*, unsigned long, unsigned long) (analogsegment.cpp:78)
   by 0x1AFDD5: pv::Session::feed_in_analog(std::shared_ptr<sigrok::Analog>) (session.cpp:1142)
   by 0x1B043C: pv::Session::data_feed_in(std::shared_ptr<sigrok::Device>, std::shared_ptr<sigrok::Packet>) (session.cpp:1187)
...
 Address 0x1cbf6b90 is 0 bytes inside a block of size 940 alloc'd
   at 0x4C2C97F: operator new[](unsigned long) (vg_replace_malloc.c:423)
   by 0x1C765E: pv::data::AnalogSegment::append_interleaved_samples(float const*, unsigned long, unsigned long) (analogsegment.cpp:78)
   by 0x1AFDD5: pv::Session::feed_in_analog(std::shared_ptr<sigrok::Analog>) (session.cpp:1142)