[Bug libdw/29434] New: Memory leak in `dwarf_getscopes`

["godlygeek at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29434

            Bug ID: 29434
           Summary: Memory leak in `dwarf_getscopes`
           Product: elfutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libdw
          Assignee: unassigned at sourceware dot org
          Reporter: godlygeek at gmail dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

Found by valgrind:

==173857== 64 bytes in 2 blocks are definitely lost in loss record 3,155 of
8,232
==173857==    at 0x480B7BB: malloc (vg_replace_malloc.c:380)
==173857==    by 0x90143DC: pc_record (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)
==173857==    by 0x9019ABC: walk_children (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)
==173857==    by 0x901974A: __libdw_visit_scopes (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)
==173857==    by 0x9019A69: walk_children (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)
==173857==    by 0x901974A: __libdw_visit_scopes (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)
==173857==    by 0x9014691: dwarf_getscopes (in
/path/to/python_extension_module.cpython-38-x86_64-linux-gnu.so)

`dwarf_getscopes` ends with:
```
  if (result > 0)
    *scopes = a.scopes;

  return result;
```

but this is incorrect, since `a.scopes` may be non-NULL even if `result` is <=
0 and is leaked in this case since no reference is retained to it. Seems like
this needs to be:
```
  if (result > 0)
    *scopes = a.scopes;
  else
    free(a.scopes);

  return result;
```

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