builtin::trim: ensure TARG is properly reset

In the reported test case, trim() sets TARG (used for the return
value), and that is then evaluated in numeric context, setting the IV
slot and pIOK and possible IOK.

These aren't reset on the second call to trim from the same entersub
OP, and since TARG is re-used it still has the IV slot and pIOK/IOK
set, and so uses the cached numeric value when evaluating TARG in
numeric context again.

Clear all but the string related flags.

Fixes Perl#22784

builtin.c

@@ -285,7 +285,7 @@ XS(XS_builtin_trim)
 
         Copy(start, SvPVX(dest), len, U8);
         SvPVX(dest)[len] = '\0';
-        SvPOK_on(dest);
+        SvPOK_only_UTF8(dest);
         SvCUR_set(dest, len);
 
         if (DO_UTF8(source))

lib/builtin.pm

@@ -1,4 +1,4 @@
-package builtin 0.016;
+package builtin 0.017;
 
 use v5.40;
 

lib/builtin.t

@@ -669,6 +669,14 @@ EOS
     }
 }
 
+# github #22784
+{
+    use builtin qw( trim );
+    sub f { 0+trim($_[0]) }
+    is(f(4), 4, "populate TARG.iv");
+    is(f(123), 123, "check TARG.IOK is reset properly");
+}
+
 # vim: tabstop=4 shiftwidth=4 expandtab autoindent softtabstop=4
 
 done_testing();