Fix memory leak

* string.c (str_make_independent_expand): free independent buffer.
  [Bug# 15935]

Co-Authored-By: luke-gru (Luke Gruber) <luke.gru@gmail.com>

string.c

@@ -2069,7 +2069,7 @@ static void
 str_make_independent_expand(VALUE str, long len, long expand, const int termlen)
 {
     char *ptr;
-    const char *oldptr;
+    char *oldptr;
     long capa = len + expand;
 
     if (len > capa) len = capa;
@@ -2088,6 +2088,9 @@ str_make_independent_expand(VALUE str, long len, long expand, const int termlen)
     if (oldptr) {
 	memcpy(ptr, oldptr, len);
     }
+    if (FL_TEST_RAW(str, STR_NOEMBED|STR_NOFREE|STR_SHARED) == STR_NOEMBED) {
+        xfree(oldptr);
+    }
     STR_SET_NOEMBED(str);
     FL_UNSET(str, STR_SHARED|STR_NOFREE);
     TERM_FILL(ptr + len, termlen);