@AT_decl_file asserts on systemtap debuginfo

[pmachata]

A strange bug in reading DW_AT_decl_file:

$ rpm -q systemtap-debuginfo
systemtap-debuginfo-2.6-3.fc21.x86_64
$ ./dwgrep/dwgrep -e 'entry (offset == 0x1306a) @AT_decl_file' /usr/lib/debug/usr/bin/stap.debug
dwgrep: /home/petr/proj/dwgrep-older/libzwerg/dwpp.hh:52: void throw_libdw(int): Assertion `dwerr != 0' failed.
Aborted (core dumped)
$ ./dwgrep/dwgrep -e 'entry (offset == 0x1306a) attribute ?AT_decl_file value' /usr/lib/debug/usr/bin/stap.debug
/usr/src/debug/systemtap-2.6/cscommon.h
/usr/src/debug/systemtap-2.6/cscommon.h

[pmachata]

Provisional fix is at: https://gist.github.com/pmachata/63426aacf01d441d9ee3
This is sub-par though in that it unnecessarily clones value_die and has no test case.