don't call null pointer if DT_INIT/DT_FINI are null

it's not clear to me why the linker even outputs these headers if they
are null, but apparently it does so. with the default startfiles, they
will never be null anyway, but this patch allows eliminating crti,
crtn, crtbegin, and crtend (leaving only crt1) if the toolchain is
using init_array/fini_array (or for a C-only, no-ctor environment).

src/ldso/dynlink.c

@@ -715,7 +715,7 @@ static void do_fini()
 			while (n--) ((void (*)(void))*--fn)();
 		}
 #ifndef NO_LEGACY_INITFINI
-		if (dyn[0] & (1<<DT_FINI))
+		if ((dyn[0] & (1<<DT_FINI)) && dyn[DT_FINI])
 			((void (*)(void))(p->base + dyn[DT_FINI]))();
 #endif
 	}
@@ -738,7 +738,7 @@ static void do_init_fini(struct dso *p)
 			fini_head = p;
 		}
 #ifndef NO_LEGACY_INITFINI
-		if (dyn[0] & (1<<DT_INIT))
+		if ((dyn[0] & (1<<DT_INIT)) && dyn[DT_INIT])
 			((void (*)(void))(p->base + dyn[DT_INIT]))();
 #endif
 		if (dyn[0] & (1<<DT_INIT_ARRAY)) {