Fix memset'ing of buffer in strrep

`strlen(buf)` is not guaranteed to return the size of the buffer, as it
might contain some null bytes in the middle.

Fixes htacg#1001

src/tmbstr.c

@@ -255,7 +255,7 @@ void TY_(strrep)(tmbstr buffer, ctmbstr str, ctmbstr rep)
         if(p)
         {
             char buf[1024];
-            memset(buf,'\0',strlen(buf));
+            memset(buf,'\0',sizeof(buf));
 
             if(buffer == p)
             {