better manage buffer for libstagefright_soft_mpeg4enc

Existing code allocated buffer, adjusted pointer to use it, and would
adjust the pointer back when it came time to free the space. The problem
was that the adjustment was based on user-supplied values and if the
user changed those values between alloc and free (which was possible),
the code ended up free()ing the wrong address.

We fix this by keeping an extra pointer -- the unmodified allocation --
which we use for the subsequent free() calls. This makes the free()
independent of any changes to values that the user provides.

Bug: 36075363
Test: ran poc against patched nyc-mr2-dev tree
Change-Id: I7013ff5883a945c4647517b2980c76a6558f23d2