zframe.c: fix type error in selftest

gcc version 4.8.2 gives:
zframe.c:708:5: error: large integer implicitly truncated to unsigned type [-Werror=overflow]
     size_t test_64bit = 0xFFFFFFFFFFFFFFFF;
czmq_prelude.h/safe_malloc(): C++-robustify the signature

silences "deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]"
Merge pull request #379 from mhaberler/master

remove some nits