backport valgrind cleanups from 6.0-engines

mysql-test/valgrind.supp:
  silence valgrind warning of memory leak in dlopen
strings/strmake.c:
  silence valgrind warning cause by strlen examining unset bytes.

mysql-test/valgrind.supp

@@ -495,3 +495,16 @@
    fun:_db_enter_
    fun:kill_server
 }
+
+#
+# Warning caused by small memory leak in threaded dlopen
+#
+
+{
+   dlopen threaded memory leak
+   Memcheck:Leak
+   fun:calloc
+   obj:*/libdl-*.so
+   fun:dlopen*
+}
+

strings/strmake.c

@@ -41,8 +41,8 @@ char *strmake(register char *dst, register const char *src, size_t length)
     write a character rather than '\0' as this makes spotting these
     problems in the results easier.
   */
-  uint n= strlen(src) + 1;
-  if (n <= length)
+  uint n= 0;
+  while (n < length && src[n++]);
   memset(dst + n, (int) 'Z', length - n + 1);
 #endif