Fix checksum failure with small isos

When the size of an iso was small enough that the fragment size was
smaller than the buffer size it wouldn't calculate or check the checksum
correctly (it would output 19 instead of 20 checksum fragments).

This fixes that by doing 2 things:
 * read the smaller of:
  - remaining bytes
  - fragment size
  - buffer size
 * When reading the last dangling bit of data add it to the checksum
   but do not write a 21st checksum fragment.

This is backwards compatible with the previous code and now you can
checksum any size iso.

libcheckisomd5.c

@@ -66,7 +66,7 @@ static enum isomd5sum_status checkmd5sum(int isofd, checkCallback cb, void *cbda
     size_t previous_fragment = 0UL;
     off_t offset = 0LL;
     while (offset < total_size) {
-        const size_t nbyte = MIN((size_t)(total_size - offset), buffer_size);
+        const size_t nbyte = MIN((size_t)(total_size - offset), MIN(fragment_size, buffer_size));
 
         ssize_t nread = read(isofd, buffer, nbyte);
         if (nread <= 0L)
@@ -89,7 +89,7 @@ static enum isomd5sum_status checkmd5sum(int isofd, checkCallback cb, void *cbda
             const size_t current_fragment = offset / fragment_size;
             const size_t fragmentsize = FRAGMENT_SUM_SIZE / info->fragmentcount;
             /* If we're onto the next fragment, calculate the previous sum and check. */
-            if (current_fragment != previous_fragment) {
+            if (current_fragment != previous_fragment && current_fragment < info->fragmentcount) {
                 if (!validate_fragment(&hashctx, current_fragment, fragmentsize,
                                        info->fragmentsums, NULL)) {
                     /* Exit immediately if current fragment sum is incorrect */

libimplantisomd5.c

@@ -109,8 +109,8 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
     const off_t fragment_size = total_size / (FRAGMENT_COUNT + 1);
     size_t previous_fragment = 0UL;
     off_t offset = 0LL;
-    while (offset < total_size) {
-        const size_t nbyte = MIN((size_t)(total_size - offset), buffer_size);
+    while (offset < total_size && previous_fragment < FRAGMENT_COUNT) {
+        const size_t nbyte = MIN((size_t)(total_size - offset), MIN(fragment_size, buffer_size));
         ssize_t nread = read(isofd, buffer, nbyte);
         if (nread <= 0L)
             break;
@@ -119,7 +119,7 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
         const size_t current_fragment = offset / fragment_size;
         const size_t fragmentsize = FRAGMENT_SUM_SIZE / FRAGMENT_COUNT;
         /* If we're onto the next fragment, calculate the previous sum and check. */
-        if (current_fragment != previous_fragment) {
+        if (current_fragment != previous_fragment && current_fragment < FRAGMENT_COUNT) {
             validate_fragment(&hashctx, current_fragment, fragmentsize, NULL, fragmentsums);
             previous_fragment = current_fragment;
         }