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>

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/test-b64_encode.c b/tests/test-b64_encode.c
new file mode 100644
index 0000000..5f011e5
--- /dev/null
+++ b/tests/test-b64_encode.c
@@ -0,0 +1,7 @@
+#include "utils.h"
+
+int main()
+{
+	b64_encode("foo", 3, NULL, 2);
+	return 0;
+}