[Ada] Pair miscount in Dynamic_HTable.Put

[Ada] Pair miscount in Dynamic_HTable.Put

This patch corrects the logic of GNAT.Dynamic_HTables.Dynamic_HTable.Put to
update the number of key-value pairs in the hash table only when the put is
adding a new pair, rather than updating the value of an existing pair.

2018-09-26  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* libgnat/g-dynhta.adb (Prepend_Or_Replace): Update the number
	of key-value pairs in the hash table only when adding a brand
	new pair.

gcc/testsuite/

	* gnat.dg/dynhash1.adb: New testcase.

From-SVN: r264623