preproc.c: Fix SIGSEV on missed %endm

In case if EOF is reached (or due to any
other case pp_cleanup is being called) we
do free "defining" macro but forgot to set
pointer to NULL itself which leads to attempt
to free memory again for this macro on further
pp_cleanup calls.

preproc.c

@@ -4804,6 +4804,7 @@ static void pp_cleanup(int pass)
         }
 
         free_mmacro(defining);
+        defining = NULL;
     }
     while (cstk)
         ctx_pop();