PnmImageParser does not check the validity of input PAM header

PnmImageParser.java directly calls tokenizer.nextToken() at line no 160, 163, 166, 169 and 172 on java.util.StringTokenizer tokenizer without checking if there are more tokens. Because tokenizer is built from the bytes string that can be invalid, this can lead to a runtime exception without a useful error message. This can be easily fixed by calling tokenizer.hasMoreTokens() before calling tokenizer.nextToken() at each line number mentioned before and throwing useful error message.