MDEV-34921 MemorySanitizer reports errors for non-debug builds

my_b_encr_write(): Initialize also block_length, and at the same time
last_block_length, so that all 128 bits can be initialized with fewer
writes. This fixes an error that was caught in the test
encryption.tempfiles_encrypted.

test_my_safe_print_str(): Skip a test that would attempt to
display uninitialized data in the test unit.stacktrace.
Previously, our CI did not build unit tests with MemorySanitizer.

handle_delayed_insert(): Remove a redundant call to pthread_exit(0),
which would for some reason cause MemorySanitizer in clang-19 to
report a stack overflow in a RelWithDebInfo build. This fixes a
failure of several tests.

Reviewed by: Vladislav Vaintroub

sql/mf_iocache_encr.cc

@@ -175,8 +175,8 @@ static int my_b_encr_write(IO_CACHE *info, const uchar *Buffer, size_t Count)
       DBUG_RETURN(info->error= -1);
     }
     crypt_data->counter= 0;
-
-    IF_DBUG(crypt_data->block_length= 0,);
+    crypt_data->block_length= 0;
+    crypt_data->last_block_length= 0;
   }
 
   do

sql/sql_insert.cc

@@ -3498,7 +3498,6 @@ pthread_handler_t handle_delayed_insert(void *arg)
     DBUG_LEAVE;
   }
   my_thread_end();
-  pthread_exit(0);
 
   return 0;
 }

unittest/mysys/stacktrace-t.c

@@ -30,7 +30,7 @@ void test_my_safe_print_str()
   memcpy(b_bss, "LEGAL", 6);
 
 #ifdef HAVE_STACKTRACE
-#ifndef __SANITIZE_ADDRESS__
+# if !defined __SANITIZE_ADDRESS__ && !__has_feature(memory_sanitizer)
   fprintf(stderr, "\n===== stack =====\n");
   my_safe_print_str(b_stack, 65535);
   fprintf(stderr, "\n===== heap =====\n");
@@ -40,15 +40,15 @@ void test_my_safe_print_str()
   fprintf(stderr, "\n===== data =====\n");
   my_safe_print_str("LEGAL", 65535);
   fprintf(stderr, "\n===== Above is a junk, but it is expected. =====\n");
-#endif /*__SANITIZE_ADDRESS__*/
+# endif
   fprintf(stderr, "\n===== Nornal length test =====\n");
   my_safe_print_str("LEGAL", 5);
   fprintf(stderr, "\n===== NULL =====\n");
   my_safe_print_str(0, 5);
-#ifndef __SANITIZE_ADDRESS__
+# ifndef __SANITIZE_ADDRESS__
   fprintf(stderr, "\n===== (const char*) 1 =====\n");
   my_safe_print_str((const char*)1, 5);
-#endif /*__SANITIZE_ADDRESS__*/
+# endif /*__SANITIZE_ADDRESS__*/
 #endif /*HAVE_STACKTRACE*/
 
   free(b_heap);