cmGeneratorExpressionParser: Fix crash on "$<" at end of input

When a previous expression in the input has already been closed,
parsing continued to the next `$<`, and when that's at the end, it
stepped past.

Source/cmGeneratorExpressionParser.cxx

@@ -224,6 +224,11 @@ void cmGeneratorExpressionParser::ParseContent(
     case cmGeneratorExpressionToken::BeginExpression:
       assert(this->it != this->Tokens.end());
       ++this->it;
+      if (this->it == this->Tokens.end()) {
+        // A '$<' at the end of the input is plain text.
+        extendText(result, this->it - 1);
+        return;
+      }
       this->ParseGeneratorExpression(result);
       return;
     case cmGeneratorExpressionToken::EndExpression:

Tests/GeneratorExpression/CMakeLists.txt

@@ -144,6 +144,8 @@ add_custom_target(check-part2 ALL
     -Dtest_incomplete_19=$<1:some,thing$<ANGLE-R>
     -Dtest_incomplete_20=$<CONFIGURATION$<ANGLE-R>
     -Dtest_incomplete_21=$<BOOL:something$<ANGLE-R>
+    -Dtest_incomplete_22=$<1:a>$<
+    -Dtest_incomplete_23=$<1:a>$<1:$<
     -Dtest_build_interface=$<BUILD_INTERFACE:build>
     -Dtest_install_interface=$<INSTALL_INTERFACE:install>
     -Dtest_target_name_1=$<TARGET_NAME:tgt,ok>

Tests/GeneratorExpression/check-part2.cmake

@@ -22,6 +22,8 @@ check(test_incomplete_18 "$<1:some,thing")
 check(test_incomplete_19 "$<1:some,thing>")
 check(test_incomplete_20 "$<CONFIGURATION>")
 check(test_incomplete_21 "$<BOOL:something>")
+check(test_incomplete_22 "a$<")
+check(test_incomplete_23 "a$<1:$<")
 check(test_build_interface "build")
 check(test_install_interface "")
 check(test_target_name_1 "tgt,ok")