ensure struct passwd fields are non-NULL in pwcopy

Android libc can return NULL pw_gecos, for example.

Author: zhangjun

misc.c

@@ -506,7 +506,7 @@ pwcopy(struct passwd *pw)
 	copy->pw_name = xstrdup(pw->pw_name);
 	copy->pw_passwd = xstrdup(pw->pw_passwd == NULL ? "*" : pw->pw_passwd);
 #ifdef HAVE_STRUCT_PASSWD_PW_GECOS
-	copy->pw_gecos = xstrdup(pw->pw_gecos);
+	copy->pw_gecos = xstrdup(pw->pw_gecos == NULL ? "" : pw->pw_gecos);
 #endif
 	copy->pw_uid = pw->pw_uid;
 	copy->pw_gid = pw->pw_gid;
@@ -517,10 +517,10 @@ pwcopy(struct passwd *pw)
 	copy->pw_change = pw->pw_change;
 #endif
 #ifdef HAVE_STRUCT_PASSWD_PW_CLASS
-	copy->pw_class = xstrdup(pw->pw_class);
+	copy->pw_class = xstrdup(pw->pw_class == NULL ? "" : pw->pw_class);
 #endif
-	copy->pw_dir = xstrdup(pw->pw_dir);
-	copy->pw_shell = xstrdup(pw->pw_shell);
+	copy->pw_dir = xstrdup(pw->pw_dir == NULL ? "" : pw->pw_dir);
+	copy->pw_shell = xstrdup(pw->pw_shell == NULL ? "" : pw->pw_shell);
 	return copy;
 }