do not use default when dynamic linker fails to open existing path file

if fopen fails for a reason other than ENOENT, we must assume the
intent is that the path file be used. failure may be due to
misconfiguration or intentional resource-exhaustion attack (against
suid programs), in which case falling back to loading libraries from
an unintended path could be dangerous.

src/ldso/dynlink.c

@@ -614,6 +614,8 @@ static struct dso *load_library(const char *name, struct dso *needed_by)
 						sys_path = "";
 					}
 					fclose(f);
+				} else if (errno != ENOENT) {
+					sys_path = "";
 				}
 			}
 			if (!sys_path) sys_path = "/lib:/usr/local/lib:/usr/lib";