[flang][unittests] Fix buffer overrun in FrontendActionTest (#84381)

When` SmallVector<char>` is used as a backing storage, it can't be
assumed to end with a \x0. When creating a `StringRef` from it, pass the
length explicitly.

This was detected by address sanitizer.

flang/unittests/Frontend/FrontendActionTest.cpp

@@ -198,7 +198,7 @@ TEST_F(FrontendActionTest, EmitLLVM) {
   EXPECT_TRUE(success);
   EXPECT_TRUE(!outputFileBuffer.empty());
 
-  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.data())
+  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.begin(), outputFileBuffer.size())
                   .contains("define void @_QQmain()"));
 }
 
@@ -227,6 +227,7 @@ TEST_F(FrontendActionTest, EmitAsm) {
   EXPECT_TRUE(success);
   EXPECT_TRUE(!outputFileBuffer.empty());
 
-  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.data()).contains("_QQmain"));
+  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.begin(), outputFileBuffer.size())
+                  .contains("_QQmain"));
 }
 } // namespace