[Bug #18154] Fix memory leak in String#initialize

String#initialize can leak memory when called on a string that is marked
with STR_NOFREE because it does not unset the STR_NOFREE flag.

string.c

@@ -1734,7 +1734,7 @@ rb_str_init(int argc, VALUE *argv, VALUE str)
                 const size_t osize = RSTRING(str)->as.heap.len + TERM_LEN(str);
                 char *new_ptr = ALLOC_N(char, (size_t)capa + termlen);
                 memcpy(new_ptr, old_ptr, osize < size ? osize : size);
-                FL_UNSET_RAW(str, STR_SHARED);
+                FL_UNSET_RAW(str, STR_SHARED|STR_NOFREE);
                 RSTRING(str)->as.heap.ptr = new_ptr;
 	    }
 	    else if (STR_HEAP_SIZE(str) != (size_t)capa + termlen) {

test/ruby/test_string.rb

@@ -105,6 +105,16 @@ PREP
 CODE
   end
 
+  # Bug #18154
+  def test_initialize_nofree_memory_leak
+    assert_no_memory_leak([], <<-PREP, <<-CODE, rss: true)
+code = proc {0.to_s.__send__(:initialize, capacity: 10000)}
+1_000.times(&code)
+PREP
+100_000.times(&code)
+CODE
+  end
+
   def test_AREF # '[]'
     assert_equal("A",  S("AooBar")[0])
     assert_equal("B",  S("FooBaB")[-1])