add long-time overdue convenience function

src/include/taler_amount_lib.h

@@ -160,6 +160,17 @@ TALER_amount_set_zero (const char *cur,
                        struct TALER_Amount *amount);
 
 
+/**
+ * Test if the given @a amount is zero.
+ *
+ * @param amount amount to compare to zero
+ * @return true if the amount is zero,
+ *         false if it is non-zero or invalid
+ */
+bool
+TALER_amount_is_zero (const struct TALER_Amount *amount);
+
+
 /**
  * Test if the given amount is valid.
  *

src/util/amount.c

@@ -241,6 +241,18 @@ TALER_amount_is_valid (const struct TALER_Amount *amount)
 }
 
 
+bool
+TALER_amount_is_zero (const struct TALER_Amount *amount)
+{
+  if (GNUNET_OK !=
+      TALER_amount_is_valid (amount))
+    return false;
+  return
+    (0 == amount->value) &&
+    (0 == amount->fraction);
+}
+
+
 /**
  * Test if @a a is valid, NBO variant.
  *