patch 8.1.1908: every popup window consumes a buffer number

Problem:    Every popup window consumes a buffer number.
Solution:   Recycle buffers only used for popup windows.  Do not list popup
            window buffers.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index aad1bae..2b2ef36 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4633,8 +4633,9 @@
 		    return _(e_invrange);
 		break;
 	    case ADDR_BUFFERS:
-		if (eap->line1 < firstbuf->b_fnum
-			|| eap->line2 > lastbuf->b_fnum)
+		// Only a boundary check, not whether the buffers actually
+		// exist.
+		if (eap->line1 < 1 || eap->line2 > get_highest_fnum())
 		    return _(e_invrange);
 		break;
 	    case ADDR_LOADED_BUFFERS: