Fix undefined behavior in _pdfioFileSeek

[vlasovsoft1979]

In some scenarios the following situation is possible in _pdfioFileSeek:

1. This condition is true: // Reading, see if we already have the data we need...
2. But pdf->bufend and pdf->bufptr are NULL

In this case we have undefined behavior because

1. (pdf->bufpos + pdf->bufend - pdf->buffer) now depends on pdf->buffer address only (but not on buffer length)
2. pdf->bufptr = pdf->buffer + (offset - pdf->bufpos); // now we have pdf->bufptr without pdf->bufend

I came across this situaltion in Linux/i386 architecture only but I think it's possible also for x64.

I propose a simple solution: to check also pdf->bufend.

[michaelrsweet]

Technically if buffer is NULL then bufend should also be NULL, but a NULL check is easy to add as insurance...