Return NULL when allocating memory of size zero

bitvector of length zero is not valid

Fixes coverity scan issues 179781

If the length resulted in l == 0 then there was no
allocation of word, which could subsequently be
dereferenced. Treat length of zero as invalid to
avoid dealing with it.

Travis fixes

Add travis and coverity badges to README.md

moved externs from crypto_kernel into its own header

fixed several compiler warnings from Firefox builds

make read only arrays const

removed variable init code which never gets used

added comments to ifdef's for easier understanding

added error in case the platform is not detected

Currently no warnings are generated on travis so by enabling
this flag we will catch when any warnings are introduced.

The host is the OS that the binary will run on,
this ensures that CPU_RISC is set to true.

Run all test with make runtest.

Add coverity scan to tavis builds

Current will run the scan on the master branch with
each build in master. If the frequency of commits
increases then can consider a coverity_scan branch.

Results can be found at:
https://scan.coverity.com/projects/cisco-libsrtp

add a big endian build on travis

This should prevent breaking big endian support.

As Ubunty 14.04 does not have stable cross compiler for
mips a 16.04 docker container is used to compile the tests.
The test are then run on host using qemu user static.

make rtp_decoder compile on MinGW

issue aclocal and autoconf in case of problems