common/amount: round tiny msat amount to 0sat

This parses < 1000msat amounts as being 0sat instead of failing.

Otherwise bcli would fail on 0-value outputs (happens on testnet for
transaction eca4cefef2a98de5df67a6630c63af5a72ab57583e0f3bf066d976f28213f447) as
parse_amount_sat() expected the amount to end with 000msat.

common/amount.c

@@ -180,7 +180,7 @@ bool parse_amount_msat(struct amount_msat *msat, const char *s, size_t slen)
 /* Valid strings:
  *  [0-9]+ => satoshi.
  *  [0-9]+sat => satoshi.
- *  [0-9]+000msat => satoshi.
+ *  [0-9]+msat => satoshi.
  *  [0-9]+.[0-9]{1,8}btc => satoshi.
  */
 bool parse_amount_sat(struct amount_sat *sat, const char *s, size_t slen)
@@ -198,8 +198,10 @@ bool parse_amount_sat(struct amount_sat *sat, const char *s, size_t slen)
 	if (!post_decimal_ptr && memeqstr(suffix_ptr, suffix_len, "sat"))
 		return from_number(&sat->satoshis, s, whole_number_len, 0);
 	if (!post_decimal_ptr && memeqstr(suffix_ptr, suffix_len, "msat")) {
-		if (!memends(s, whole_number_len, "000", strlen("000")))
-			return false;
+		if (!memends(s, whole_number_len, "000", strlen("000"))) {
+			*sat = AMOUNT_SAT(0);
+			return true;
+		}
 		return from_number(&sat->satoshis, s, whole_number_len - 3, 0);
 	}
 	if (post_decimal_ptr && memeqstr(suffix_ptr, suffix_len, "btc"))