fix (. (* p) x) expanding to *(p).x

Was an fmt-c-writer bug: unary-op must parenthize itself, and not it's
arg

sex-fmt-c.scm

@@ -1015,9 +1015,24 @@
                              (cat nl (make-space (+ 2 (fmt-col st))) str " "))
                    st))))))))))
 
+  ;; `-' arrives as the symbol binary minus uses, and so with binary
+  ;; precedence: `(+ (- b) b)' asked under that spelling comes out
+  ;; `(-b) + b'.
+  (define (unary-operator op)
+    (case op
+      ((-) 'unary-)
+      ((+) 'unary+)
+      ((*) 'unary-*)
+      ((&) 'unary-&)
+      (else op)))
+
+  ;; Parenthesises the whole expression rather than the operand: the
+  ;; other way round, `(. (* p) x)' came out `*(p).x', which C reads as
+  ;; `*(p.x)'.
   (define (c-unary-op op x)
     (c-wrap-stmt
-     (cat (display-to-string op) (c-maybe-paren op (c-expr x)))))
+     (c-maybe-paren (unary-operator op)
+                    (cat (display-to-string op) (c-expr x)))))
 
   ;; some convenience definitions