Fix crashes/freezes in VSS stripping code (#913)

* Fix VSS stripping behavior when headers span >1 network block.

When VSS header data straddles multiple network blocks (i.e. got <
bsidsize in bfile_write_vss_strip()), the pointer arithmatic when
memcpy()ing the next block is incorrect.

Since sid is a pointer to a structure, each addition by 1 increases
the pointer address by sizeof(*sid); therefore, when sidlen > 0, the
memcpy() unintentionally overwrites part of the struct BFILE it lives
within or the memory situated after it.

Casting sid to a char* in the memcpy() causes the addition to advance
the pointer by single bytes as intended.

* Don't hang when stripping VSS data from files with an empty stream.

When stripping VSS data, an empty stream that has additional data
following it will currently cause an infinite loop in
bfile_write_vss_strip().

When encountering an empty stream, attempt to start reading a new VSS
header instead.

---------

Co-authored-by: Ben Winslow <[email protected]>

src/bfile.c

@@ -47,14 +47,19 @@ static ssize_t bfile_write_vss_strip(struct BFILE *bfd, void *buf, size_t count)
 			size_t sidlen=bsidsize-mysid->needed_s;
 			int got=min(mysid->needed_s, mycount);
 
-			memcpy(sid+sidlen, cp, got);
+			memcpy((char *)sid+sidlen, cp, got);
 
 			cp+=got;
 			mycount-=got;
 			mysid->needed_s-=got;
 
-			if(!mysid->needed_s)
+			if(!mysid->needed_s) {
 				mysid->needed_d=sid->Size+sid->dwStreamNameSize;
+				// If the stream is completely empty, start
+				// reading a new VSS header.
+				if (!mysid->needed_d)
+					mysid->needed_s=bsidsize;
+			}
 		}
 		if(mysid->needed_d)
 		{