Genex: Fix crash on strings concatenated on LINK_GROUP

When a string is concatenated to the beginning and/or end of a
`$<LINK_GROUP:...>` generator expression without a semicolon, the
internal group marker is disrupted, leading to a crash after
evaluation.

Source/cmComputeLinkDepends.cxx

@@ -1076,7 +1076,11 @@ void cmComputeLinkDepends::AddLinkEntries(cm::optional<size_t> depender_index,
     size_t dependee_index;
 
     if (cmHasPrefix(item.AsStr(), LG_END) && cmHasSuffix(item.AsStr(), '>')) {
-      assert(group);
+      if (!group) {
+        // The matching group start was not recognized, e.g., because
+        // other text was concatenated to it. Ignore the stray end.
+        continue;
+      }
       dependee_index = group->first;
       if (group->second) {
         this->GroupItems.emplace(group->first, std::move(groupItems));

Tests/RunCMake/GenEx-LINK_GROUP/RunCMakeTest.cmake

@@ -29,6 +29,7 @@ run_cmake(nested-incompatible-features2)
 run_cmake(circular-dependencies1)
 run_cmake(circular-dependencies2)
 run_cmake(only-targets)
+run_cmake(concatenated-group-begin)
 
 # usage of LINK_LIBRARY with LINK_GROUP
 run_cmake(incompatible-library-features1)

Tests/RunCMake/GenEx-LINK_GROUP/concatenated-group-begin.cmake

@@ -0,0 +1,15 @@
+enable_language(C)
+
+set(CMAKE_C_LINK_GROUP_USING_feat_SUPPORTED TRUE)
+set(CMAKE_C_LINK_GROUP_USING_feat "--start" "--stop")
+
+add_library(dep SHARED empty.c)
+
+# Text concatenated before the genex hides the start of the group.
+add_library(lib1 SHARED empty.c)
+target_link_libraries(lib1 PRIVATE "x$<LINK_GROUP:feat,dep>")
+
+add_library(iface INTERFACE)
+target_link_libraries(iface INTERFACE "x$<LINK_GROUP:feat,dep>")
+add_library(lib2 SHARED empty.c)
+target_link_libraries(lib2 PRIVATE iface)