bb_uuencode: fix obscure case where we were using data past last byte of source

(fixes testsuite failure)

bb_uuencode                                          154     160      +6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 6/0)                 Total: 6 bytes
   text    data     bss     dec     hex filename
 770284    1096   11228  782608   bf110 busybox_old
 770288    1096   11228  782612   bf114 busybox_unstripped

1 files changed, 22 insertions, 16 deletions

diff --git a/libbb/uuencode.c b/libbb/uuencode.c
index 08fe3f366..57d1cbfd2 100644
--- a/libbb/uuencode.c
+++ b/libbb/uuencode.c
@@ -39,27 +39,33 @@ const char bb_uuenc_tbl_std[65] = {
  * buffer of at least 1+BASE64_LENGTH(length) bytes.
  * where BASE64_LENGTH(len) = (4 * ((LENGTH + 2) / 3))
  */
-void bb_uuencode(char *store, const void *src, int length, const char *tbl)
+void bb_uuencode(char *p, const void *src, int length, const char *tbl)
 {
-	int i;
 	const unsigned char *s = src;
-	char *p = store;
 
-	/* Transform the 3x8 bits to 4x6 bits, as required by base64.  */
-	for (i = 0; i < length; i += 3) {
+	/* Transform the 3x8 bits to 4x6 bits */
+	while (length > 0) {
+		unsigned s1, s2;
+
+		/* Are s[1], s[2] valid or should be assumed 0? */
+		s1 = s2 = 0;
+		length -= 3; /* can be >=0, -1, -2 */
+		if (length != -2) {
+			s1 = s[1];
+			if (length != -1)
+				s2 = s[2];
+		}
 		*p++ = tbl[s[0] >> 2];
-		*p++ = tbl[((s[0] & 3) << 4) + (s[1] >> 4)];
-		*p++ = tbl[((s[1] & 0xf) << 2) + (s[2] >> 6)];
-		*p++ = tbl[s[2] & 0x3f];
+		*p++ = tbl[((s[0] & 3) << 4) + (s1 >> 4)];
+		*p++ = tbl[((s1 & 0xf) << 2) + (s2 >> 6)];
+		*p++ = tbl[s2 & 0x3f];
 		s += 3;
 	}
-	/* Pad the result if necessary...  */
-	if (i == length + 1) {
-		*(p - 1) = tbl[64];
-	}
-	else if (i == length + 2) {
-		*(p - 1) = *(p - 2) = tbl[64];
-	}
-	/* ...and zero-terminate it.  */
+	/* Zero-terminate */
 	*p = '\0';
+	/* If length is -2 or -1, pad last char or two */
+	while (length) {
+		*--p = tbl[64];
+		length++;
+	}
 }