base64: fix possible null pointer dereference

base64: fix possible null pointer dereference

clang-10 analyzer reports following:

 base64.c:325:20: warning: Array access (from variable 'target') results in a null pointer dereference
                 target[tarindex] = 0;
                 ~~~~~~           ^

and prepared test case confirms it:

 Invalid write of size 1
    at 0x4E4463F: b64_decode (base64.c:325)
    by 0x40088C: test_invalid_inputs (tests/test-base64.c:26)
    by 0x40088C: main (tests/test-base64.c:32)
  Address 0x1 is not stack'd, malloc'd or (recently) free'd

 Process terminating with default action of signal 11 (SIGSEGV)
  Access not within mapped region at address 0x1
    at 0x4E4463F: b64_decode (base64.c:325)
    by 0x40088C: test_invalid_inputs (tests/test-base64.c:26)
    by 0x40088C: main (tests/test-base64.c:32)

Signed-off-by: Petr Štetiar <ynezz@true.cz>