Bugs found with Clang static analyzer

[elshize]

Logic error	Dereference of null pointer	include/pisa/bit_vector.hpp	skip	426	29	View Report	Report Bug	Open File
Logic error	Dereference of null pointer	include/pisa/bit_vector.hpp	next	410	22	View Report	Report Bug	Open File
Logic error	Division by zero	include/pisa/codec/compact_elias_fano.hpp	offsets	29	18	View Report	Report Bug	Open File
Logic error	Result of operation is garbage or undefined	include/pisa/codec/integer_codes.hpp	read_gamma	26	22	View Report	Report Bug	Open File
Logic error	Result of operation is garbage or undefined	include/pisa/bit_vector.hpp	get_bits	239	25	View Report	Report Bug	Open File
Logic error	Result of operation is garbage or undefined	include/pisa/codec/integer_codes.hpp	read_gamma	26	24	View Report	Report Bug	Open File

Details are in the program output: report.zip

We can ignore the ones from FastPFor; it's not a big deal and I've already created a PR in the lib repo: https://github.com/lemire/FastPFor/pull/65

[elshize]

Note that the above results are incomplete, we should re-run scan-build tool to get the full list.

[elshize]

No longer seeing any errors in our code.