seedrng: avoid needless runtime strlen() call

- Avoid needless runtime strlen() call, bloating binary.
- Replace failed seed string with series of nulls.

function                                             old     new   delta
.rodata                                           108350  108338     -12
static.seedrng_prefix                                 26       -     -26
seedrng_main                                        1000     948     -52
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/2 up/down: 0/-90)             Total: -90 bytes
   text	   data	    bss	    dec	    hex	filename
 975919	   4227	   1816	 981962	  efbca	busybox_old
 975829	   4227	   1816	 981872	  efb70	busybox_unstripped

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
1 file changed