[UB] Avoid a really broken call to realloc that would later result in

a bad call to memcpy.

When we only have a buffer from one of the two reparse calls, we can
just return that buffer rather than going through the realloc/memcpy
dance.

Found with UBsan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243950 91177308-0d34-0410-b5e6-96231b3b80d8

tools/c-index-test/c-index-test.c

@@ -255,6 +255,17 @@ static int parse_remapped_files_with_try(int try_idx,
   if (ret)
     return ret;
 
+  if (num_unsaved_files_no_try_idx == 0) {
+    *unsaved_files = unsaved_files_try_idx;
+    *num_unsaved_files = num_unsaved_files_try_idx;
+    return 0;
+  }
+  if (num_unsaved_files_try_idx == 0) {
+    *unsaved_files = unsaved_files_no_try_idx;
+    *num_unsaved_files = num_unsaved_files_no_try_idx;
+    return 0;
+  }
+
   *num_unsaved_files = num_unsaved_files_no_try_idx + num_unsaved_files_try_idx;
   *unsaved_files
     = (struct CXUnsavedFile *)realloc(unsaved_files_no_try_idx,