Change the limit on the input size for b2a_base64 to what will fit in

memory, rather than the standard's 57.

This fixes SF bug #473009.

Modules/binascii.c

@@ -135,7 +135,9 @@ static char table_a2b_base64[] = {
 };
 
 #define BASE64_PAD '='
-#define BASE64_MAXBIN 57	/* Max binary chunk size (76 char line) */
+
+/* Max binary chunk size; limited only by available memory */
+#define BASE64_MAXBIN (INT_MAX/2 - sizeof(PyStringObject))
 
 static unsigned char table_b2a_base64[] =
 "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";