tweaks: elide a call of strlen() for every row

For a normal file (without overlong lines) the strlen() wasn't much
of a problem.  But when there are very long lines, it wasted time
counting stuff that wouldn't be displayed on the current row anyway,
and reserved *far* too much memory for the displayable string.

Problem existed since commit cf0eed6c from five years ago that traded
a continuous comparison (of the used space with the reserved space)
against a one-time big reservation up front involving a strlen().
In retrospect that was not a good trade-off when softwrapping.

The extra check (charwidth == 0) is incurred only by characters that
have their high bit set, so the average file (with only ASCII) is not
affected by this -- it just loses an unneeded call of strlen().

src/winio.c

@@ -1717,8 +1717,12 @@ char *display_string(const char *text, size_t column, size_t span,
 		/* The index of the first character that the caller wishes to show. */
 	size_t start_col = wideness(text, start_index);
 		/* The actual column where that first character starts. */
-	char *converted;
-		/* The expanded string we will return. */
+	size_t stowaways = 20;
+		/* The number of zero-width characters for which to reserve space. */
+	size_t allocsize = (COLS + stowaways) * MAXCHARLEN + 1;
+		/* The amount of memory to reserve for the displayable string. */
+	char *converted = nmalloc(allocsize);
+		/* The displayable string we will return. */
 	size_t index = 0;
 		/* Current position in converted. */
 	size_t beyond = column + span;
@@ -1726,9 +1730,6 @@ char *display_string(const char *text, size_t column, size_t span,
 
 	text += start_index;
 
-	/* Allocate enough space for converting the relevant part of the line. */
-	converted = nmalloc(strlen(text) * (MAXCHARLEN + tabsize) + 1);
-
 #ifndef NANO_TINY
 	if (span > HIGHEST_POSITIVE) {
 		statusline(ALERT, "Span has underflowed -- please report a bug");
@@ -1842,6 +1843,13 @@ char *display_string(const char *text, size_t column, size_t span,
 		/* Determine whether the character takes zero, one, or two columns. */
 		charwidth = wcwidth(wc);
 
+		/* Watch the number of zero-widths, to keep ample memory reserved. */
+		if (charwidth == 0 && --stowaways == 0) {
+			stowaways = 40;
+			allocsize += stowaways * MAXCHARLEN;
+			converted = nrealloc(converted, allocsize);
+		}
+
 #ifdef __linux__
 		/* On a Linux console, skip zero-width characters, as it would show
 		 * them WITH a width, thus messing up the display.  See bug #52954. */