fix skslc crashes uncovered by fuzz testing

src/sksl/SkSLIRGenerator.cpp

Index: src/sksl/SkSLIRGenerator.cpp
diff --git a/src/sksl/SkSLIRGenerator.cpp b/src/sksl/SkSLIRGenerator.cpp
index 6307087edd9d4f97233d1ea16f20aed848dc5b04..9a42379065e05bb1ecc355fc35361471dae1b78d 100644
--- a/src/sksl/SkSLIRGenerator.cpp
+++ b/src/sksl/SkSLIRGenerator.cpp
@@ -1228,6 +1228,7 @@ std::unique_ptr<Expression> IRGenerator::convertSuffixExpression(
                                                                      *newType));
             } else {
                 fErrors.error(expression.fPosition, "'[]' must follow a type name");
+                return nullptr;
             }
         }
         case ASTSuffix::kCall_Kind: {