vis: fix command malfunction triggered by special cursor position

At the command prompt, commands were not recognized properly when the
cursor was placed on a delimiting character (:, ?, /) while pressing
enter.

Fix martanne#653

vis-prompt.c

@@ -65,11 +65,8 @@ static const char *prompt_enter(Vis *vis, const char *keys, const Arg *arg) {
 			pattern = "^(/|\\?)";
 		if (pattern && regex && text_regex_compile(regex, pattern, REG_EXTENDED|REG_NEWLINE) == 0) {
 			char c;
-			size_t prev;
-			if (text_byte_get(txt, pos, &c) && (c == ':' || c == '/' || c == '?'))
-				prev = pos;
-			else
-				prev = text_search_backward(txt, pos, regex);
+			size_t end = text_line_end(txt, pos);
+			size_t prev = text_search_backward(txt, end, regex);
 			if (prev > pos)
 				prev = EPOS;
 			size_t next = text_search_forward(txt, pos, regex);