fix skslc crashes uncovered by fuzz testing

src/sksl/SkSLParser.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #include "stdio.h"
 #include "SkSLParser.h"
 #include "SkSLToken.h"
@@ skipping 790 matching lines @@
     }
     if (!this->expect(Token::LPAREN, "'('")) {
         return nullptr;
     }
     std::unique_ptr<ASTStatement> initializer;
     Token nextToken = this->peek();
     switch (nextToken.fKind) {
         case Token::SEMICOLON: 
             this->nextToken();
             break;
-        case Token::CONST:
+        case Token::CONST: {
+            std::unique_ptr<ASTVarDeclarations> vd = this->varDeclarations();
+            if (!vd) {
+                return nullptr;
+            }
             initializer = std::unique_ptr<ASTStatement>(new ASTVarDeclarationStatement(
-                                                                          this->varDeclarations()));
+                                                                                    std::move(vd)));
             break;
-        case Token::IDENTIFIER: 
+        }
+        case Token::IDENTIFIER: {
             if (this->isType(nextToken.fText)) {
+                std::unique_ptr<ASTVarDeclarations> vd = this->varDeclarations();
+                if (!vd) {
+                    return nullptr;
+                }
                 initializer = std::unique_ptr<ASTStatement>(new ASTVarDeclarationStatement(
-                                                                          this->varDeclarations()));
+                                                                                    std::move(vd)));
                 break;
             }
-            // fall through
+        } // fall through
         default:
             initializer = this->expressionStatement();
     }
     std::unique_ptr<ASTExpression> test;
     if (this->peek().fKind != Token::SEMICOLON) {
         test = this->expression();
         if (!test) {
             return nullptr;
         }
     }
@@ skipping 603 matching lines @@
 bool Parser::identifier(std::string* dest) {
     Token t;
     if (this->expect(Token::IDENTIFIER, "identifier", &t)) {
         *dest = t.fText; 
         return true;
     }
     return false;
 }
 
 } // namespace