Fixes #3085: dotnet new crashes if template contains #ifdef

Problem was that in `if` statement above took path where `tokens` doesn't add any token, reason for that is that there was no `whitespace` or `brace` token after it, instead just "Literal" which is not fully parsed yet and can't be added to `tokens`, at least thats my understanding...
What is weird about current logic is... that `#ifBAR` is same as `#if BAR`... Easy fix would be to do add space after `if`, but might cause other issues... I think lets not invest too much enery into this and lets just fix crash if user types `#iffoo bar` so it doesn't crash...

Author: DavidKarlas

src/Microsoft.TemplateEngine.Core/Expressions/Cpp/CppStyleEvaluatorDefinition.cs

@@ -114,7 +114,7 @@ public static bool Evaluate(IProcessorState processor, ref int bufferLength, ref
             }
 
             int braceDepth = 0;
-            if (tokens[0].Family == TokenFamily.OpenBrace)
+            if (tokens.Count > 0 && tokens[0].Family == TokenFamily.OpenBrace)
             {
                 ++braceDepth;
             }

test/Microsoft.TemplateEngine.Core.UnitTests/ConditionalTests.CStyleEvaluator.cs

@@ -1596,5 +1596,34 @@ public void VerifyEmitStrayToken()
             //  pretend it's false because the inputs and outputs are the same
             Verify(Encoding.UTF8, output, false, value, expected);
         }
+
+        [Theory]
+        [InlineData("foo", true)]
+        [InlineData("foo", false)]
+        [InlineData("def", true)]
+        [InlineData("def", false)]
+        // dotnet new crashes if template contains #ifdef
+        // https://github.com/dotnet/templating/issues/3085
+        public void VerifyMisstypedIfTokenDoesntCrash(string varName, bool varValue)
+        {
+            string value = @"#ifdef
+GAGA
+#endif
+";
+            string expected = varName == "def" && varValue ? @"GAGA
+" : "";
+
+            byte[] valueBytes = Encoding.UTF8.GetBytes(value);
+            MemoryStream input = new MemoryStream(valueBytes);
+            MemoryStream output = new MemoryStream();
+
+            VariableCollection vc = new VariableCollection()
+            {
+                [varName] = varValue
+            };
+            IProcessor processor = SetupCStyleNoCommentsProcessor(vc);
+            processor.Run(input, output, 999);
+            Verify(Encoding.UTF8, output, true, value, expected);
+        }
     }
 }