fix paren placing in fmt

Prior to the fix, the Sex code
`(while (!= EOF (= c (getc f))) ...)` expanded wrongly to
`while (EOF != c = getc(f)) { ...`, instead of
`while (EOF != (c = getc(f))) { ...`

sex-fmt-c.scm

@@ -112,7 +112,7 @@
     (lambda (st)
       ((fmt-let 'op op
                 (if (and (c-op<= (fmt-op st) op)
-                         (not (vector? st)))
+                         (not (vector? x)))
                     (c-paren x)
                     x))
        st)))