sam: fix + address

Previoulsy + would not advance to the next line when the cursor was
on the first character of a line. This should fix the +- idiom, i.e.

 :+-x/foo/c/bar/

performs a substitution on the current line.

sam.c

@@ -980,7 +980,7 @@ static Filerange address_line_evaluate(Address *addr, File *file, Filerange *ran
 	size_t start = range->start, end = range->end, line;
 	if (sign > 0) {
 		char c;
-		if (end > 0 && text_byte_get(txt, end-1, &c) && c == '\n')
+		if (start < end && end > 0 && text_byte_get(txt, end-1, &c) && c == '\n')
 			end--;
 		line = text_lineno_by_pos(txt, end);
 		line = text_pos_by_lineno(txt, line + offset);