MDEV-16777: galera.galera_gra_log fails with File ...GRA_*.log not found error

snprintf returns the number of bytes it wrote (or would have written) NOT
counting the \0 terminal character.
The buffer size it accepts as argument DOES COUNT the \0 character.
Pass the right parameter value.

sql/wsrep_binlog.cc

@@ -326,11 +326,16 @@ void wsrep_dump_rbr_buf(THD *thd, const void* rbr_buf, size_t buf_len)
     WSREP_ERROR("snprintf error: %d, skipping dump.", len);
     return;
   }
+  /*
+    len doesn't count the \0 end-of-string. Use len+1 below
+    to alloc and pass as an argument to snprintf.
+  */
 
   char *filename= (char *)malloc(len+1);
-  int len1= snprintf(filename, len, "%s/GRA_%ld_%lld.log",
+  int len1= snprintf(filename, len+1, "%s/GRA_%ld_%lld.log",
                     wsrep_data_home_dir, thd->thread_id,
                     (long long)wsrep_thd_trx_seqno(thd));
+
   if (len > len1)
   {
     WSREP_ERROR("RBR dump path truncated: %d, skipping dump.", len);
@@ -469,7 +474,10 @@ void wsrep_dump_rbr_buf_with_header(THD *thd, const void *rbr_buf,
   int len= snprintf(NULL, 0, "%s/GRA_%ld_%lld_v2.log",
                     wsrep_data_home_dir, thd->thread_id,
                     thd_trx_seqno);
-
+  /*
+    len doesn't count the \0 end-of-string. Use len+1 below
+    to alloc and pass as an argument to snprintf.
+  */
   char *filename;
   if (len < 0 || !(filename= (char*)malloc(len+1)))
   {
@@ -477,7 +485,7 @@ void wsrep_dump_rbr_buf_with_header(THD *thd, const void *rbr_buf,
     DBUG_VOID_RETURN;
   }
 
-  int len1= snprintf(filename, len, "%s/GRA_%ld_%lld_v2.log",
+  int len1= snprintf(filename, len+1, "%s/GRA_%ld_%lld_v2.log",
                      wsrep_data_home_dir, thd->thread_id,
                      thd_trx_seqno);