At various points through the function librsvg_decode_frame, errors are
returned from immediately without deallocating any allocated structs.
This patch both fixes those leaks, and also fixes the use of functions
that are deprecated since librsvg version 2.52.0. The older calls are
still used, guarded by #ifdefs while the newer replacements are used if
librsvg >= 2.52.0. One of the deprecated functions is used as a check
for the configure shell script, so it was replaced with a different
function.
Signed-off-by: Leo Izen <leo.izen@gmail.com>