o BUG BUG fix (sic!): readPgf(..., indices=NULL) should never have worked, cf. Issue #2.

The code R_affx_clf_pgf_parser.cpp#L99-L102:

```
    while (pgf->next_probeset() == TSV_OK) {
        if (nProbesets >= LENGTH(indices)) {
            break;
        }
```

assumed `LENGTH(indices)` returns 0 when indices is `NULL`, whereas in reality
it returns a (random garbage?) large number.

EXAMPLE:

```
fcn_LENGTH <- cfunction(signature(indices="integer"), '
  printf("n=%d\\n", LENGTH(indices));
  return(indices);
')

> fcn_LENGTH(1:10)
n=10
[1]  1  2  3  4  5  6  7  8  9 10

> fcn_LENGTH(NULL)
n=1247112
NULL
```
whereas `length(x)` works as expected:

```
fcn_length <- cfunction(signature(indices="integer"), '
  printf("n=%d\\n", length(indices));
  return(indices);
')

> fcn_length(1:10)
n=10
[1]  1  2  3  4  5  6  7  8  9 10

> fcn_length(NULL)
n=0
NULL
```