Initialise target_t members on error paths

Initialise target_t members on error paths

This is a no-op from the point of view of the actual code flow, but
not doing so can trigger false positives in some versions of GCC (4.9
from Jessie) at high optimisation levels (-O3). For an interesting
discussion of why this happens at `-O3` vs. at `-O2`, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60165.

Closes #13.