hstore_plperl: Add CHECK_FOR_INTERRUPTS() in reference-unwinding loop.

Add CHECK_FOR_INTERRUPTS() to the while loop in plperl_to_hstore()
that dereferences chains of Perl references, so that a circular
reference (e.g. $x = \$x) can be cancelled by the user instead of
spinning indefinitely.  (We looked at detecting such circular
references, but it seems more trouble than it's worth.)

This is a follow-up to da82fbb8f, which fixed the same issue in
SV_to_JsonbValue() in jsonb_plperl.

Author: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAJ7c6TPbjkzUk4qJ5dHvDNEz0hBuFue3A-XWz_=897z+BC+z8A@mail.gmail.com
Backpatch-through: 14

contrib/hstore_plperl/hstore_plperl.c

@@ -2,6 +2,7 @@
 
 #include "fmgr.h"
 #include "hstore/hstore.h"
+#include "miscadmin.h"
 #include "plperl.h"
 
 PG_MODULE_MAGIC_EXT(
@@ -110,7 +111,15 @@ plperl_to_hstore(PG_FUNCTION_ARGS)
 
 	/* Dereference references recursively. */
 	while (SvROK(in))
+	{
+		/*
+		 * It's possible for circular references to make this an infinite
+		 * loop.  Checking for such a situation seems like much more trouble
+		 * than it's worth, but let's provide a way to break out of the loop.
+		 */
+		CHECK_FOR_INTERRUPTS();
 		in = SvRV(in);
+	}
 
 	/* Now we must have a hash. */
 	if (SvTYPE(in) != SVt_PVHV)