Genex: Don't segfault on $<FOO,>

Treat the comma as part of the identifier here. It will later not
resolve to a generator expression and the user gets a proper error
message.

Source/cmGeneratorExpressionParser.cxx

@@ -88,7 +88,15 @@ void cmGeneratorExpressionParser::ParseGeneratorExpression(
   while(this->it->TokenType != cmGeneratorExpressionToken::EndExpression
       && this->it->TokenType != cmGeneratorExpressionToken::ColonSeparator)
     {
+    if (this->it->TokenType == cmGeneratorExpressionToken::CommaSeparator)
+      {
+      extendText(identifier, this->it);
+      ++this->it;
+      }
+    else
+      {
       this->ParseContent(identifier);
+      }
     if (this->it == this->Tokens.end())
       {
       break;

Tests/RunCMake/GeneratorExpression/BadZero-stderr.txt

@@ -6,3 +6,12 @@ CMake Error at BadZero.cmake:2 \(add_custom_target\):
   \$<0> expression requires a parameter.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
++
+CMake Error at BadZero.cmake:2 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<0,>
+
+  Expression did not evaluate to a known generator expression
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)

Tests/RunCMake/GeneratorExpression/BadZero.cmake

@@ -1,4 +1,5 @@
 
 add_custom_target(check ALL COMMAND check
   $<0>
+  $<0,>
   VERBATIM)