Fix possible dereference null pointer (PQprint)

Hi.

In the function *PQprint*, the variable po->fieldName
can be NULL.
See the checks a few lines up.

for (numFieldName = 0;
po->fieldName && po->fieldName[numFieldName];
numFieldName++)

So, I think that must be checked, when used,
in the loop below.

best regards,
Ranier Vilela