Avoid an infloop when we run out of memory

Avoid an infloop when we run out of memory

* src/alloc.c (garbage_collect_1): Don't bother saving and
restoring the echo-area message if we are GC'ing after running out
of memory.  This avoids an infloop due to repeated attempts to
allocate memory for the cons cell needed to save the message,
which signals the memory-full error, which attempts to save the
echo-area message, which signals memory-full again, etc.