data: fix memory leak when printing data to memory

When print() writes to memory, libyang allocates the output buffer.
ly_out_free() is called without the destroy flag, so the buffer stays
allocated, and print() copies it into a python string but never frees
it. Each call leaks the printed data.

Free the buffer after the copy. With a 10 kB leaf printed 10000 times,
the RSS growth is:

  before: 100156 kB
  after:  0 kB

Note that print_mem() and Module.print() already free their buffer.

Fixes: 806be4c ("Port to libyang 2")
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>

Author: samuel-gauthier

libyang/data.py

@@ -723,10 +723,12 @@ def print(
             ret = lib.lyd_print_tree(out_data[0], self.cdata, fmt, flags)
 
         lib.ly_out_free(out_data[0], ffi.NULL, 0)
-        if ret != lib.LY_SUCCESS:
-            raise self.context.error("failed to write data")
-
-        return c2str(buf[0], decode=True)
+        try:
+            if ret != lib.LY_SUCCESS:
+                raise self.context.error("failed to write data")
+            return c2str(buf[0], decode=True)
+        finally:
+            lib.free(buf[0])
 
     def print_mem(
         self,