TFM: fix big endian reading a zero length buffer

Bail early as big endian implementation doesn't handle it.

wolfcrypt/src/tfm.c

@@ -3697,6 +3697,10 @@ int fp_read_unsigned_bin(fp_int *a, const unsigned char *b, int c)
   /* zero the int */
   fp_zero (a);
 
+  if (c == 0) {
+      return FP_OKAY;
+  }
+
   /* if input b excess max, then truncate */
   if (c > 0 && (word32)c > maxC) {
      int excess = (c - maxC);