Merge #990: Add comment on length checks when parsing ECDSA sigs

e02f313 Add comment on length checks when parsing ECDSA sigs (Tim Ruffing)

Pull request description:

  I claim the check can be removed but I don't want to touch this
  stable and well-tested code.

  On the way, we fix grammar in another comment.

ACKs for top commit:
  sipa:
    ACK e02f313
  RandyMcMillan:
    ACK e02f313

Tree-SHA512: f82691a8f5db82a1e9683e52ce8e952ebd56b476a2817c5a876ce4638254b7b4ac93175318fb59598ed5532f33433951d75afea03724ef4419c3e1bd12ca8c20

src/ecdsa_impl.h

@@ -66,8 +66,7 @@ static int secp256k1_der_read_len(size_t *len, const unsigned char **sigp, const
     }
     if (lenleft > sizeof(size_t)) {
         /* The resulting length would exceed the range of a size_t, so
-         * certainly longer than the passed array size.
-         */
+         * it is certainly longer than the passed array size. */
         return 0;
     }
     while (lenleft > 0) {
@@ -76,7 +75,9 @@ static int secp256k1_der_read_len(size_t *len, const unsigned char **sigp, const
         lenleft--;
     }
     if (*len > (size_t)(sigend - *sigp)) {
-        /* Result exceeds the length of the passed array. */
+        /* Result exceeds the length of the passed array.
+           (Checking this is the responsibility of the caller but it
+           can't hurt do it here, too.) */
         return 0;
     }
     if (*len < 128) {