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>

base64.c

@@ -65,6 +65,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+
+#include "assert.h"
 #include "utils.h"
 
 static const char Base64[] =
@@ -144,6 +146,8 @@ int b64_encode(const void *_src, size_t srclength,
 	u_char output[4];
 	size_t i;
 
+	assert(dest && targsize > 0);
+
 	while (2 < srclength) {
 		input[0] = *src++;
 		input[1] = *src++;
@@ -208,6 +212,8 @@ int b64_decode(const void *_src, void *dest, size_t targsize)
 	state = 0;
 	tarindex = 0;
 
+	assert(dest && targsize > 0);
+
 	while ((ch = (unsigned char)*src++) != '\0') {
 		if (isspace(ch))	/* Skip whitespace anywhere. */
 			continue;

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.

tests/test-b64_decode.c

@@ -0,0 +1,7 @@
+#include "utils.h"
+
+int main()
+{
+	b64_decode("Zg==", NULL, 2);
+	return 0;
+}

tests/test-b64_encode.c

@@ -0,0 +1,7 @@
+#include "utils.h"
+
+int main()
+{
+	b64_encode("foo", 3, NULL, 2);
+	return 0;
+}