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>

3 files changed, 26 insertions, 0 deletions

diff --git a/tests/cram/test_base64.t b/tests/cram/test_base64.t
index 4f8809f..0a7a9d5 100644
--- a/tests/cram/test_base64.t
+++ b/tests/cram/test_base64.t
@@ -19,3 +19,15 @@ check that base64 is producing expected results:
   4 foob
   5 fooba
   6 foobar
+
+check that b64_encode and b64_decode assert invalid input
+
+  $ alias check="egrep '(dumped|Assertion)' | sed 's;.*\(b64_.*code\).*\(Assertion.*$\);\1: \2;' | LC_ALL=C sort"
+
+  $ test-b64_decode 2>&1 | check
+  Aborted (core dumped)
+  b64_decode: Assertion `dest && targsize > 0' failed.
+
+  $ test-b64_encode 2>&1 | check
+  Aborted (core dumped)
+  b64_encode: Assertion `dest && targsize > 0' failed.
diff --git a/tests/test-b64_decode.c b/tests/test-b64_decode.c
new file mode 100644
index 0000000..4798fa8
--- /dev/null
+++ b/tests/test-b64_decode.c
@@ -0,0 +1,7 @@
+#include "utils.h"
+
+int main()
+{
+	b64_decode("Zg==", NULL, 2);
+	return 0;
+}
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;
+}