Fix crash for single rest on class declaration (#315)

I thought I didn't need to check the length but forgot that the rest argument is not in the list for class declaration. Now it doesn't crash anymore and there's a test...

src/printer.js

@@ -1741,7 +1741,7 @@ function printFunctionParams(path, print, options) {
   }
 
   const lastParam = util.getLast(path.getValue().params);
-  const canHaveTrailingComma = lastParam.type !== "RestElement" && !fun.rest;
+  const canHaveTrailingComma = !(lastParam && lastParam.type === "RestElement") && !fun.rest;
 
   return concat([
     "(",

tests/rest/__snapshots__/jsfmt.spec.js.snap

@@ -10,6 +10,8 @@ function f(
   superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
   ...args
 ) {}
+
+declare class C { f(...superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong): void; }
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 declare class C {
   f(
@@ -23,5 +25,11 @@ function f(
   ...args
 ) {
 }
+
+declare class C {
+  f(
+    ...superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong
+  ): void,
+}
 "
 `;

tests/rest/trailing-commas.js

@@ -9,3 +9,5 @@ function f(
   superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
   ...args
 ) {}
+
+declare class C { f(...superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong): void; }