Ensure line isn't valid is it's not read in parse_string()

Thus appears to have been the root cause of the infinite loop/memory
issue.  Basically, line was never replaced if it didn't match
^{[0-9]+}$ and didn't result in a number when passed to parseInt()
This resulted in an infinite loop that kept pushing zero-length
strings into an array.

exec/imapservice.js

@@ -716,6 +716,9 @@ function parse_command(line)
 				ret=client.socket.recv(len);
 				line=client.socket.recvline(10240, 1800);
 			}
+			else {
+				line = undefined;
+			}
 			return(ret);
 		}