Fix out of bound in rtrim()

When the string with length zero, it might be coredump because of
out-of-bound in rtrim().

Reported-By: CPUBarbecue

Author: japinli

pg_node2graph.cc

@@ -364,7 +364,7 @@ rtrim(const string& str)
 {
 	size_t len = str.size();
 
-	while (isspace(str[len - 1])) {
+	while (len > 0 && isspace(str[len - 1])) {
 		len--;
 	}