gdb_packet: Check vasprintf return value harder and skip free().

* Initialize *buf on stack.
* If vasprintf fails to allocate, *buf is old value, not a pointer on heap;
  freeing it crashes firmware.
* Emit a constant string to logging channel for builds with diagnostics.

src/gdb_packet.c

@@ -325,13 +325,17 @@ void gdb_put_notification(const char *const packet, const size_t size)
 void gdb_putpacket_f(const char *const fmt, ...)
 {
  va_list ap;
- char *buf;
+ char *buf = NULL;
 
  va_start(ap, fmt);
  const int size = vasprintf(&buf, fmt, ap);
- if (size > 0)
+ if (size < 0) {
+ /* Heap exhaustion. Report with puts() elsewhere. */
+ DEBUG_ERROR("gdb_putpacket_f: vasprintf failed\n");
+ } else {
  gdb_putpacket(buf, size);
- free(buf);
+ free(buf);
+ }
  va_end(ap);
 }