Avoid use of uninitialized in init_dpi_end

If conn == NULL or display == NULL, init_dpi() jumps to init_dpi_end
before (declaring and) initializing resource. In init_dpi_end, there
is a free(resource) call conditionally on resource != NULL, so this
may lead to a bogus free. Found by clang -Wsometimes-uninitialized.