patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/regexp_bt.c b/src/regexp_bt.c
index 0ed3711..fef66e4 100644
--- a/src/regexp_bt.c
+++ b/src/regexp_bt.c
@@ -470,7 +470,7 @@
 
     num_complex_braces = 0;
     regnpar = 1;
-    vim_memset(had_endbrace, 0, sizeof(had_endbrace));
+    CLEAR_FIELD(had_endbrace);
 #ifdef FEAT_SYN_HL
     regnzpar = 1;
     re_has_z = 0;