seekable decompression fixes (#2594)

* seekable_format: fix from-file reading (not in-memory)

It tries to check the buffer boundary, but there is no buffer for
from-file reading.

* seekable_decompression: break when ZSTD_seekable_decompress() returns zero

* seekable_decompression_mem: break when ZSTD_seekable_decompress() returns zero

* seekable_format: cap the offset+len up to the last dOffset

This will allow to read the whole file w/o gotting corruption error if
the offset is more then the data left in file, i.e.:

    $ ./seekable_compression seekable_compression.c 8192 | head
    $ zstd -cdq seekable_compression.c.zst | wc -c
    4737

Before this patch:

    $ ./seekable_decompression seekable_compression.c.zst 0 10000000 | wc -c
    ZSTD_seekable_decompress() error : Corrupted block detected
    0

After:

    $ ./seekable_decompression seekable_compression.c.zst 0 10000000 | wc -c
    4737

contrib/seekable_format/examples/seekable_decompression.c

@@ -99,6 +99,9 @@ static void decompressFile_orDie(const char* fname, off_t startOffset, off_t end
 
     while (startOffset < endOffset) {
         size_t const result = ZSTD_seekable_decompress(seekable, buffOut, MIN(endOffset - startOffset, buffOutSize), startOffset);
+        if (!result) {
+            break;
+        }
 
         if (ZSTD_isError(result)) {
             fprintf(stderr, "ZSTD_seekable_decompress() error : %s \n",

contrib/seekable_format/examples/seekable_decompression_mem.c

@@ -104,6 +104,9 @@ static void decompressFile_orDie(const char* fname, off_t startOffset, off_t end
 
     while (startOffset < endOffset) {
         size_t const result = ZSTD_seekable_decompress(seekable, buffOut, MIN(endOffset - startOffset, buffOutSize), startOffset);
+        if (!result) {
+            break;
+        }
 
         if (ZSTD_isError(result)) {
             fprintf(stderr, "ZSTD_seekable_decompress() error : %s \n",