alsamidisrc: Fix a memory leak when setting the "ports" property

alsamidisrc: Fix a memory leak when setting the "ports" property

When setting the "ports" property the value is duplicated but it's not
freed when the elements stops.

Reported by Valgrind (example run with "alsamidisrc ports=128:0"):

  6 bytes in 1 blocks are definitely lost in loss record 30 of 1,911
     at 0x4C2BBEF: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
     by 0x5411528: g_malloc (gmem.c:94)
     by 0x542A9FE: g_strdup (gstrfuncs.c:363)
     by 0x775211E: gst_alsa_midi_src_set_property (gstalsamidisrc.c:284)
     by 0x5184A4D: object_set_property (gobject.c:1439)
     by 0x5184A4D: g_object_setv (gobject.c:2245)
     by 0x51859DD: g_object_set_property (gobject.c:2529)
     by 0x4F0474C: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
     by 0x4F065C8: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
     by 0x4F07557: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
     by 0x4EFE3EE: gst_parse_launch_full (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
     by 0x4EFE673: gst_parse_launchv_full (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)

https://bugzilla.gnome.org/show_bug.cgi?id=787683