encoding/base64: Fix compilation warning with ARM GCC 13

Error: repos/apache-mynewt-core/encoding/base64/src/base64.c:
    In function 'base64_decoder_go':
repos/apache-mynewt-core/encoding/base64/src/base64.c:261:9: error:
    'memcpy' specified size between 2147483653 and 4294967295 exceeds
     maximum object size 2147483647 [-Werror=stringop-overflow=]
  261 |  memcpy(&dec->buf[dec->buf_len], &dec->src[src_off], read_len);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

encoding/base64/src/base64.c

@@ -42,6 +42,7 @@
 #include <string.h>
 #include <limits.h>
 #include <stdio.h>
+#include <assert.h>
 
 #include <os/mynewt.h>
 #include <base64/base64.h>
@@ -233,6 +234,7 @@ base64_decoder_go(struct base64_decoder *dec)
         }
 
         /* Account for possibility of partial token from previous call. */
+        assert(dec->buf_len < 4);
         read_len = 4 - dec->buf_len;
 
         /* Detect invalid input. */