Fix some minor infelicities in ecpg's pgtypeslib: (1) `pstr' must be

Fix some minor infelicities in ecpg's pgtypeslib: (1) `pstr' must be
non-NULL in this function, so there is no need to check for it (2) we
should check the return value of pgtypes_strdup(). Patch from Eric
Astor at EnterpriseDB, with slight cleanup by myself, per a report
from the Coverity tool.