allow disabling animation playback via -A 0

ref: https://codeberg.org/nsxiv/nsxiv/issues/404

etc/nsxiv.1

@@ -37,6 +37,9 @@ manager.
 .BI "\-A, \-\-framerate " FRAMERATE
 Play animations with a constant frame rate set to
 .IR FRAMERATE .
+If
+.I FRAMERATE
+is set to 0 then animation playback is disabled.
 .TP
 .B "\-a, \-\-animate"
 Play animations of multi-frame images.

options.c

@@ -171,10 +171,11 @@ void parse_options(int argc, char **argv)
 			exit(EXIT_FAILURE);
 		case 'A':
 			n = strtol(op.optarg, &end, 0);
-			if (*end != '\0' || n <= 0 || n > INT_MAX)
+			if (*end != '\0' || n < 0 || n > INT_MAX)
 				error(EXIT_FAILURE, 0, "Invalid framerate: %s", op.optarg);
 			_options.framerate = n;
-			/* fall through */
+			_options.animate = n > 0;
+			break;
 		case 'a':
 			_options.animate = true;
 			break;