patch 8.2.3942: Coverity reports a possible memory leak

Problem:    Coverity reports a possible memory leak.
Solution:   Free the array if allocation fails.
diff --git a/src/insexpand.c b/src/insexpand.c
index c547892..07ac71d 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -3193,6 +3193,8 @@
     num_matches = expand_spelling(lnum, compl_pattern, &matches);
     if (num_matches > 0)
 	ins_compl_add_matches(num_matches, matches, p_ic);
+    else
+	vim_free(matches);
 #endif
 }