Fix a compilation failure on svref of a symbol macro

 Introduced in 0892423b (faster SVREF and (SETF SVREF) compilation)

 Thanks to James M. Lawrence for the quick report, with a reduced
 test case.
Stop emitting references to inexistant #n= forms in the pretty printer

 * Special logic was introduced in 2003 to avoid pprinting backquote-comma
   forms as ", foo" when ",foo" is unambiguous; the bug has likely been
   around since then.

 * Reported by Douglas Katzman on launchpad, and reduced by James M. Lawrence
   (lp#1161218)