fix use after free in font caching algorithm

Current font caching algorithm contains a use after free error. A font
removed from `frc` might be still listed in `wx.specbuf`. It will lead
to a crash inside `XftDrawGlyphFontSpec()`.

Steps to reproduce:
$ st -f 'Misc Tamsyn:scalable=false'
$ curl https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-demo.txt

Of course, result depends on fonts installed on a system and fontconfig.
In my case, I'm getting consistent segfaults with different fonts.

I replaced a fixed array with a simple unbounded buffer with a constant
growth rate. Cache starts with a capacity of 0, gets increments by 16,
and never shrinks. On my machine after `cat UTF-8-demo.txt` buffer
reaches a capacity of 192. During casual use capacity stays at 0.

x.c

@@ -226,8 +226,9 @@ typedef struct {
 } Fontcache;
 
 /* Fontcache is an array now. A new font will be appended to the array. */
-static Fontcache frc[16];
+static Fontcache *frc = NULL;
 static int frclen = 0;
+static int frccap = 0;
 static char *usedfont = NULL;
 static double usedfontsize = 0;
 static double defaultfontsize = 0;
@@ -1244,12 +1245,14 @@ xmakeglyphfontspecs(XftGlyphFontSpec *specs, const Glyph *glyphs, int len, int x
 					fcpattern, &fcres);
 
 			/*
-			 * Overwrite or create the new cache entry.
+			 * Allocate memory for the new cache entry.
 			 */
-			if (frclen >= LEN(frc)) {
-				frclen = LEN(frc) - 1;
-				XftFontClose(xw.dpy, frc[frclen].font);
-				frc[frclen].unicodep = 0;
+			if (frclen >= frccap) {
+				frccap += 16;
+				if (!frc)
+					frc = xmalloc(frccap * sizeof(Fontcache));
+				else
+					frc = xrealloc(frc, frccap * sizeof(Fontcache));
 			}
 
 			frc[frclen].font = XftFontOpenPattern(xw.dpy,