add malformed fn form match case in fmt-c-writer

Spent like 10 minutes trying to understand why my fn pointer is being
fucked up in test case. Turned out I was missing return type, and
whole fn pointer type fall through down to defaut case.

fmt-c-writer.scm

@@ -126,6 +126,8 @@
          `(%array ,(walk-type (maybe-unwrap-type array-type)))))
     (('fn arglist ret-type)
      `(%fun ,(walk-type ret-type) ,(walk-arglist arglist)))
+    (('fn . _)
+     (assert #f "Malformed function type form"))
 
     ;; Special case: nested structs/unions
     ((or ('struct . _)