MDEV-25109 Server crashes in sp_name::sp_name upon invalid data in mysql.proc

don't try to lowercase a db name if it's zero-length.
(empty_lex_str is not writable, even db.str[0]=0 will fail)

mysql-test/main/lowercase_table.result

@@ -148,3 +148,19 @@ drop table t1;
 #
 # End of 10.2 tests
 #
+#
+# MDEV-25109 Server crashes in sp_name::sp_name upon invalid data in mysql.proc
+#
+call mtr.add_suppression("Stored routine ''.'': invalid value in column");
+insert ignore into mysql.proc () values ();
+Warnings:
+Warning	1364	Field 'param_list' doesn't have a default value
+Warning	1364	Field 'returns' doesn't have a default value
+Warning	1364	Field 'body' doesn't have a default value
+Warning	1364	Field 'comment' doesn't have a default value
+show function status;
+ERROR 42000: Incorrect routine name ''
+delete from mysql.proc where name = '';
+#
+# End of 10.3 tests
+#

mysql-test/main/lowercase_table.test

@@ -126,3 +126,16 @@ drop table t1;
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+--echo #
+--echo # MDEV-25109 Server crashes in sp_name::sp_name upon invalid data in mysql.proc
+--echo #
+call mtr.add_suppression("Stored routine ''.'': invalid value in column");
+insert ignore into mysql.proc () values ();
+--error ER_SP_WRONG_NAME
+show function status;
+delete from mysql.proc where name = '';
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/sp_head.h

@@ -111,7 +111,7 @@ public:
           bool use_explicit_name)
     : Database_qualified_name(db, name), m_explicit_name(use_explicit_name)
   {
-    if (lower_case_table_names && m_db.str)
+    if (lower_case_table_names && m_db.length)
       m_db.length= my_casedn_str(files_charset_info, (char*) m_db.str);
   }