field: Static-assert that int args affecting magnitude are constant

See #1001.

src/util.h

@@ -51,6 +51,19 @@ static void print_buf_plain(const unsigned char *buf, size_t len) {
 #  define SECP256K1_INLINE inline
 # endif
 
+/** Assert statically that expr is an integer constant expression, and run stmt.
+ *
+ * Useful for example to enforce that magnitude arguments are constant.
+ */
+#define ASSERT_INT_CONST_AND_DO(expr, stmt) do { \
+    switch(42) { \
+        case /* ERROR: integer argument is not constant */ expr: \
+            break; \
+        default: ; \
+    } \
+    stmt; \
+} while(0)
+
 typedef struct {
     void (*fn)(const char *text, void* data);
     const void* data;