libbacktrace: don't exit Mach-O dyld library loop on one failure

* macho.c (backtrace_initialize) [HAVE_MACH_O_DYLD_H]: Don't exit
	loop if we can't find debug info for one shared library.

For #85

macho.c

@@ -1268,7 +1268,7 @@ backtrace_initialize (struct backtrace_state *state, const char *filename,
       mff = macho_nodebug;
       if (!macho_add (state, name, d, 0, NULL, base_address, 0,
 		      error_callback, data, &mff, &mfs))
-	return 0;
+	continue;
 
       if (mff != macho_nodebug)
 	macho_fileline_fn = mff;