[fcstr.c] Embed a static 64-byte buffer in FcStrBuf

Reduces number of mallocs called from FcStrBufChar in my small test
from 900 down to 6.

src/fcint.h

@@ -313,6 +313,7 @@ typedef struct _FcStrBuf {
     FcBool  failed;
     int	    len;
     int	    size;
+    FcChar8 buf_static[16 * sizeof (void *)];
 } FcStrBuf;
 
 struct _FcCache {

src/fcstr.c

@@ -710,11 +710,18 @@ FcUtf16Len (const FcChar8   *string,
 void
 FcStrBufInit (FcStrBuf *buf, FcChar8 *init, int size)
 {
-    buf->buf = init;
+    if (init)
+    {
+	buf->buf = init;
+	buf->size = size;
+    } else
+    {
+	buf->buf = buf->static_buf;
+	buf->size = sizeof (buf->static_buf);
+    }
     buf->allocated = FcFalse;
     buf->failed = FcFalse;
     buf->len = 0;
-    buf->size = size;
 }
 
 void