Prevents unintended free in finish_merged_header when n_targets == 0

[daviesrob]

finish_merged_header reallocs the target_name and target_len arrays to
be exactly n_targets in size. This caused a problem when n_targets == 0
as realloc(ptr, 0) frees ptr. To stop this from happening, make sure we
always allocate at least one element.

Fixes #495

[jmarshall]

Thanks for tracking this down! I think the bug is more about the interplay between those lines and the later error-handling lines

hdr->target_name = target_name ? target_name : merged_hdr->target_name;
hdr->target_len  = target_len ? target_len : merged_hdr->target_len;

which are wrong when realloc(merged_hdr->target_name,0) frees merged_hdr->target_name and returns NULL. (Which is a glibc behaviour not shared by BSD libc btw…) So I fixed it differently in 90dcd90.