data: fix memory leak when reading anydata values

lyd_any_value_str() returns a string allocated by libyang, and the
caller must free it. value() copies it into a python string but never
frees it, so each call leaks the printed content.

Free it after the copy. With a 10 kB anyxml value read 10000 times,
the RSS growth is:

  before: 97812 kB
  after:  0 kB

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

Author: samuel-gauthier

libyang/data.py

@@ -1175,7 +1175,10 @@ def value(self, fmt: str = "xml"):
         ret = lib.lyd_any_value_str(self.cdata, data_format(fmt), anystr)
         if ret != lib.LY_SUCCESS:
             raise self.context.error("cannot get data")
-        return c2str(anystr[0])
+        try:
+            return c2str(anystr[0])
+        finally:
+            lib.free(anystr[0])
 
 
 # -------------------------------------------------------------------------------------
@@ -1186,7 +1189,10 @@ def value(self, fmt: str = "xml"):
         ret = lib.lyd_any_value_str(self.cdata, data_format(fmt), anystr)
         if ret != lib.LY_SUCCESS:
             raise self.context.error("cannot get data")
-        return c2str(anystr[0])
+        try:
+            return c2str(anystr[0])
+        finally:
+            lib.free(anystr[0])
 
 
 # -------------------------------------------------------------------------------------