mime: replace small memcpy with loop

mime: replace small memcpy with loop

To address:

      In file included from /usr/include/string.h:535,
                 from suricata-common.h:108,
                 from util-decode-mime.c:26:
In function ‘memcpy’,
    inlined from ‘ProcessBase64Remainder’ at util-decode-mime.c:1201:13:
/usr/include/mipsel-linux-gnu/bits/string_fortified.h:29:10: warning: ‘__builtin_memcpy’ forming offset 4 is out of the bounds [0, 4] of object ‘block’ with type ‘uint8_t[4]’ {aka ‘unsigned char[4]’} [-Warray-bounds=]
   29 |   return __builtin___memcpy_chk (__dest, __src, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   30 |                                  __glibc_objsize0 (__dest));
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~
util-decode-mime.c: In function ‘ProcessBase64Remainder’:
util-decode-mime.c:1174:13: note: ‘block’ declared here
 1174 |     uint8_t block[B64_BLOCK];
      |             ^~~~~

Copy data should be <= 4 bytes.