Return the correct values for (log -0w0) and (log 0w0). Previously we

returned something like #c(-715 pi) and -715.

 * src/code/irrat-dd.lisp:
   * Handle -0w0 and 0w0 specially in dd-%log.

 * tests/trac.lisp:
   * Add a test for this in trac.lisp.