Several fixes to dumper. CID:107,108,206

Several fixes to dumper. CID:107,108,206
	NEGATIVE_RETURNS, REVERSE_INULL

	* src/dumper.c: Make pdump_length an off_t to fix CID:108
	* src/dumper.c (pdump_register_sub): move strlen inside the if to only do it when str!=NULL CID:206
	* src/dumper.c (pdump): Make sure the fds are >=0 and abort otherwise due to the early nature of the call.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>