fix skslc crashes uncovered by fuzz testing

src/sksl/SkSLIRGenerator.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 "SkSLIRGenerator.h"
 
 #include "limits.h"
@@ skipping 1210 matching lines @@
                 return this->convertIndex(std::move(base), *expr);
             } else if (base->fKind == Expression::kTypeReference_Kind) {
                 const Type& oldType = ((TypeReference&) *base).fValue;
                 Type* newType = new Type(oldType.name() + "[]", Type::kArray_Kind, oldType, 
                                          -1);
                 fSymbolTable->takeOwnership(newType);
                 return std::unique_ptr<Expression>(new TypeReference(fContext, base->fPosition, 
                                                                      *newType));
             } else {
                 fErrors.error(expression.fPosition, "'[]' must follow a type name");
+                return nullptr;
             }
         }
         case ASTSuffix::kCall_Kind: {
             auto rawArguments = &((ASTCallSuffix&) *expression.fSuffix).fArguments;
             std::vector<std::unique_ptr<Expression>> arguments;
             for (size_t i = 0; i < rawArguments->size(); i++) {
                 std::unique_ptr<Expression> converted = 
                         this->convertExpression(*(*rawArguments)[i]);
                 if (!converted) {
                     return nullptr;
@@ skipping 94 matching lines @@
         case Expression::kIndex_Kind:
             this->markWrittenTo(*((IndexExpression&) expr).fBase);
             break;
         default:
             fErrors.error(expr.fPosition, "cannot assign to '" + expr.description() + "'");
             break;
     }
 }
 
 }