Zero fill realloc'd memory for _nad_realloc.

Several pieces of code in nad.c call on NAD_SAFE to realloc memory and use the memory without initializing it. One particular example is in nad_insert_nad() where phantom namespaces were being added because the code checked for lprefix for element namespaces and reading in values from uninitialized memory returned from realloc. This change is a bit conservative but a safer choice to avoid future silent corruptions.