strbuf: Invalidate (only) when stolen

strbuf: Invalidate (only) when stolen

The main for strbuf_steal() to free() on error was to allow the caller
to pass the NULL up the stack with just a return call to
strbuf_steal().

However this is error-prone and surprising that the buffer is still
invalidated on error. Provide an strbuf cleanup attribute that can be
used for the same purpose and make sure that when the string is stolen,
it's set to NULL, so there's no dangling pointer around.

Since we run the  testsuite with AddressSanitizer, a simple test can be
added to make sure the stolen string becomes valid when the attribute is
used.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/239