compiler: off-by-one error in forall_syms

test/wundef found this, but only sometimes.

src/compiler/symtab.h

@@ -34,7 +34,7 @@ extern int num_user_syms;
 
 #define	forall_syms(p) \
 	for ((p) = well_known; (p) != user_syms+num_user_syms; \
-	    (p) = (p) == well_known+num_well_known ? user_syms : (p)+1)
+	    (p) = (p) == well_known+num_well_known-1 ? user_syms : (p)+1)
 
 void symtab_init(void);
 struct sym *unique(const char *s);