gstmodule: _remap: fix memory leak

The memory in info is a pointer to GstMemory, it is unmapped with gst_buffer_unmap() or gst_memory_unmap()

An extra copy of GstMemory in not needed in this case, because it is always present until the call of _unmap()

The extra copy cause memory leak in some cenarios with python plugins.

This piece of code can be used to check the memory leak on master branch, to run this:

mkdir python
cd python
python map_bug.py

map_bug.py

'''
GST_PLUGIN_PATH=.. GST_DEBUG=python:4 gst-launch-1.0 videotestsrc ! map_bug ! videorate ! fakevideosink
'''

import gi
gi.require_version('GstBase', '1.0')

from gi.repository import Gst, GObject, GstBase
Gst.init(None)

#
# Simple MapBug element created entirely in python
#
class MapBug(GstBase.BaseTransform):
    __gstmetadata__ = ('MapBug Python','Transform', \
                      'Simple identity element written in python', 'Marianna S. Buschle')

    __gsttemplates__ = (Gst.PadTemplate.new("src",
                                           Gst.PadDirection.SRC,
                                           Gst.PadPresence.ALWAYS,
                                           Gst.Caps.new_any()),
                       Gst.PadTemplate.new("sink",
                                           Gst.PadDirection.SINK,
                                           Gst.PadPresence.ALWAYS,
                                           Gst.Caps.new_any()))
    def __init__(self):
        '''
        https://www.kernel.org/doc/Documentation/filesystems/proc.txt
        '''
        self.labels = ('size:', ' resident:', ' shared:', ' trs:', ' lrs:', ' drs:', ' dt:')

    def do_transform_ip(self, buffer):
        info = buffer.map(Gst.MapFlags.READ | Gst.MapFlags.WRITE)
        buffer.unmap(info)
        self.process_memory_status()
        return Gst.FlowReturn.OK

    def process_memory_status(self):
        with open('/proc/self/statm') as f:
            statm = list(f.read().split())
        info = "".join(i + j for i, j in zip(self.labels, statm))
        Gst.info(info)

GObject.type_register(MapBug)
__gstelementfactory__ = ("map_bug", Gst.Rank.NONE, MapBug)