XOR Encryption with "Padded Key"

I read that XOR encryption can be considered very safe as long as two conditions are fulfilled.
1. The length of the key is as long (or longer) than the data
2. The key is not following a notable pattern (i.e. it's a random jumble of characters)

In that case, how about this: Before the XOR operations you use the (short) key to generate a seed for a Random Number Generator. You then use this Generator to create characters which are added to the end of your key until it's as long as the data you want to encrypt.
Then you use this new key to XOR the data.

I have tested this and it does seem to have no problem working as intended (it can encrypt and decrypt without corruption of the data).
My question is how "secure" such an encryption would be. Anyone have an estimate of how hard it'd be to break/decrypt that data?