Partially rewrite switch_xml_set_attr to fix memory leaks, uninitialized
argument values and use-after free warnings from Clang static analyzer.
Fixes these problems:
- Add some comments and a new variable such that the code can more
easily be audited / understood.
- Always clear SWITCH_XML_DUP flag even if an error occurred to prevent
free()'ing static strings on future invocations.
- Keep the attribute list in a consistent state even if one of the
memory allocation fails.
- Keep allocation metadata in a consistent state when shrinking of the
attribute lists fails. Previously the metadata was not updated,
resulting in a wrong mapping from attributes to allocation flags.
- Fix memory leaks when allocations fail.
Previous behavior: invalid memory accesses are possible after a memory
allocation failure, previous attributes may be lost.
New behavior: attributes list is always valid, a new attribute is either
set (or not), attributes can always be removed.
b683199688