libio: Free backup area when it not required (BZ#22415)

libio: Free backup area when it not required (BZ#22415)

Some libio operations fail to correctly free the backup area (created
by _IO_{w}default_pbackfail on unget{w}c) resulting in either invalid
buffer free operations or memory leaks.

For instance, on the example provided by BZ#22415 a following
fputc after a fseek to rewind the stream issues an invalid free on
the buffer.  It is because although _IO_file_overflow correctly
(from fputc) correctly calls _IO_free_backup_area, the
_IO_new_file_seekoff (called by fseek) updates the FILE internal
pointers without first free the backup area (resulting in invalid
values in the internal pointers).

The wide version also shows an issue, but instead of accessing invalid
pointers it leaks the backup memory on fseek/fputwc operation.

Checked on x86_64-linux-gnu and i686-linux-gnu.

	* libio/Makefile (tests): Add tst-bz22415.
	(tst-bz22415-ENV): New rule.
	(generated): Add tst-bz22415.mtrace and tst-bz22415.check.
	(tests-special): Add tst-bz22415-mem.out.
	($(objpfx)tst-bz22415-mem.out): New rule.
	* libio/fileops.c (_IO_new_file_seekoff): Call _IO_free_backup_area
	in case of a successful seek operation.
	* libio/wfileops.c (_IO_wfile_seekoff): Likewise.
	(_IO_wfile_overflow): Call _IO_free_wbackup_area in case a write
	buffer is required.
	* libio/tst-bz22415.c: New test.