Fix a segmentation fault occurring in the mod_perl2 test suite. (Closes: #475498)

Upstream change #33807 backported from blead.

[perl #52740]
diff --git a/sv.c b/sv.c
index 718e305..fe36438 100644
--- a/sv.c
+++ b/sv.c
@@ -3557,8 +3557,10 @@ Perl_sv_setsv_flags(pTHX_ SV *dstr, register SV *sstr, I32 flags)
 		GvMULTI_on(dstr);
 		return;
 	    }
-	    glob_assign_glob(dstr, sstr, dtype);
-	    return;
+	    if (isGV_with_GP(sstr)) {
+		glob_assign_glob(dstr, sstr, dtype);
+		return;
+	    }
 	}
 
 	if (dtype >= SVt_PV) {