Add missing CHECK_FOR_INTERRUPTS in lseg_inside_poly

Apparently, there are bugs in this code that cause it to loop endlessly.
That bug still needs more research, but in the meantime it's clear that
the loop is missing a check for interrupts so that it can be cancelled
timely.

Backpatch to 9.1 -- this has been missing since 49475aab8d0d.

src/backend/utils/adt/geo_ops.c

@@ -20,6 +20,7 @@
 #include <ctype.h>
 
 #include "libpq/pqformat.h"
+#include "miscadmin.h"
 #include "utils/builtins.h"
 #include "utils/geo_decls.h"
 
@@ -3894,6 +3895,8 @@ lseg_inside_poly(Point *a, Point *b, POLYGON *poly, int start)
 	{
 		Point	   *interpt;
 
+		CHECK_FOR_INTERRUPTS();
+
 		s.p[1] = poly->p[i];
 
 		if (on_ps_internal(t.p, &s))