Fix memory leaks in libdtrace

[trix_juniper.net]

Summary

dt_open.c dt_provmod_open
When realloc fails, the original pointer is still valid, so it must be freed.

dt_open.c dt_vopen
When the malloc of dtp fails, the memory/structure previously assigned to provmod must be freed. dt_provmod_destroy handles the case when provmod == NULL so there is no need for special handling of jumping over the setup of provmod.

dt_strtab.c dt_strtab_insert
When the call to dt_strtab_copyin fails, it is necessary to free the locally allocated memory assigned to 'hp' before returning early.

Test Plan

Run clang's static analyzer scan-build before to reproduce the problem, run scan-build after to verify resolution of the problem.