don't crash when asked to read from a file that doesn't exist (issue #6786)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@14462 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_readfile.c

@@ -95,7 +95,7 @@ static int readfile_exec(struct ast_channel *chan, void *data)
 		}
 	}
 
-	returnvar = ast_read_textfile(file);
+	if ((returnvar = ast_read_textfile(file))) {
 		if (len > 0) {
 			if (len < strlen(returnvar))
 				returnvar[len]='\0';
@@ -104,6 +104,7 @@ static int readfile_exec(struct ast_channel *chan, void *data)
 		}
 		pbx_builtin_setvar_helper(chan, varname, returnvar);
 		free(returnvar);
+	}
 	LOCAL_USER_REMOVE(u);
 	return res;
 }