examples/http2-down/upload: add error checks

If `index.html` does not exist in the directory from which the example
is invoked, the fopen(upload, "rb") invocation in `setup` would fail,
returning NULL.  This value is subsequently passed as the FILE* argument
of the `fread` invocation in the `read_callback` function, which is the
actual cause of the crash (apparently `fread` assumes that argument to
be non-null).

In addition, mitigate some possible crashes of similar origin.

Closes #5463

docs/examples/http2-download.c

@@ -26,6 +26,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <errno.h>
 
 /* somewhat unix-specific */
 #include <sys/time.h>
@@ -33,6 +34,7 @@
 
 /* curl stuff */
 #include <curl/curl.h>
+#include <curl/mprintf.h>
 
 #ifndef CURLPIPE_MULTIPLEX
 /* This little trick will just make sure that we don't enable pipelining for
@@ -146,9 +148,14 @@ static void setup(struct transfer *t, int num)
 
   hnd = t->easy = curl_easy_init();
 
-  snprintf(filename, 128, "dl-%d", num);
+  curl_msnprintf(filename, 128, "dl-%d", num);
 
   t->out = fopen(filename, "wb");
+  if(!t->out) {
+    fprintf(stderr, "error: could not open file %s for writing: %s\n",
+            filename, strerror(errno));
+    exit(1);
+  }
 
   /* write to this file */
   curl_easy_setopt(hnd, CURLOPT_WRITEDATA, t->out);