Plug memory leak when reloading config file.

The absolute path to config file was not pfreed. There are probably more
small leaks here and there in the config file reload code and assign hooks,
and in practice no-one reloads the config files frequently enough for it to
be a problem, but this one is trivial enough that might as well fix it.

Backpatch to 9.3 where the leak was introduced.

Author: hlinnaka

src/backend/utils/misc/guc-file.l

@@ -409,6 +409,7 @@ ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
 				ConfigVariable **head_p,
 				ConfigVariable **tail_p)
 {
+	char	   *abs_path;
 	bool		OK = true;
 	FILE	   *fp;
 
@@ -426,28 +427,29 @@ ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
 		return false;
 	}
 
-	config_file = AbsoluteConfigLocation(config_file,calling_file);
-	fp = AllocateFile(config_file, "r");
+	abs_path = AbsoluteConfigLocation(config_file, calling_file);
+	fp = AllocateFile(abs_path, "r");
 	if (!fp)
 	{
 		if (strict)
 		{
 			ereport(elevel,
 					(errcode_for_file_access(),
 					 errmsg("could not open configuration file \"%s\": %m",
-							config_file)));
+							abs_path)));
 			return false;
 		}
 
 		ereport(LOG,
 				(errmsg("skipping missing configuration file \"%s\"",
-						config_file)));
+						abs_path)));
 		return OK;
 	}
 
-	OK = ParseConfigFp(fp, config_file, depth, elevel, head_p, tail_p);
+	OK = ParseConfigFp(fp, abs_path, depth, elevel, head_p, tail_p);
 
 	FreeFile(fp);
+	pfree(abs_path);
 
 	return OK;
 }