7zip: fix crash when parsing certain archives

Fuzzing with CRCs disabled revealed that a call to get_uncompressed_data()
would sometimes fail to return at least 'minimum' bytes. This can cause
the crc32() invocation in header_bytes to read off into invalid memory.

A specially crafted archive can use this to cause a crash.

An ASAN trace is below, but ASAN is not required - an uninstrumented
binary will also crash.

==7719==ERROR: AddressSanitizer: SEGV on unknown address 0x631000040000 (pc 0x7fbdb3b3ec1d bp 0x7ffe77a51310 sp 0x7ffe77a51150 T0)
==7719==The signal is caused by a READ memory access.
    #0 0x7fbdb3b3ec1c in crc32_z (/lib/x86_64-linux-gnu/libz.so.1+0x2c1c)
    #1 0x84f5eb in header_bytes (/tmp/libarchive/bsdtar+0x84f5eb)
    #2 0x856156 in read_Header (/tmp/libarchive/bsdtar+0x856156)
    #3 0x84e134 in slurp_central_directory (/tmp/libarchive/bsdtar+0x84e134)
    #4 0x849690 in archive_read_format_7zip_read_header (/tmp/libarchive/bsdtar+0x849690)
    #5 0x5713b7 in _archive_read_next_header2 (/tmp/libarchive/bsdtar+0x5713b7)
    #6 0x570e63 in _archive_read_next_header (/tmp/libarchive/bsdtar+0x570e63)
    #7 0x6f08bd in archive_read_next_header (/tmp/libarchive/bsdtar+0x6f08bd)
    #8 0x52373f in read_archive (/tmp/libarchive/bsdtar+0x52373f)
    #9 0x5257be in tar_mode_x (/tmp/libarchive/bsdtar+0x5257be)
    #10 0x51daeb in main (/tmp/libarchive/bsdtar+0x51daeb)
    #11 0x7fbdb27cab96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #12 0x41dd09 in _start (/tmp/libarchive/bsdtar+0x41dd09)

This was primarly done with afl and FairFuzz. Some early corpus entries
may have been generated by qsym.
skip_stream(struct archive_read *a, size_t skip_bytes)
{
	struct _7zip *zip = (struct _7zip *)a->format->data;
	const void *p;
	int64_t skipped_bytes;
	size_t bytes = skip_bytes;

	if (zip->folder_index == 0) {
		/*
		 * Optimization for a list mode.
		 * Avoid unnecessary decoding operations.
		 */
		zip->si.ci.folders[zip->entry->folderIndex].skipped_bytes
		    += skip_bytes;
		return (skip_bytes);
	}

	while (bytes) {
		skipped_bytes = read_stream(a, &p, bytes, 0);
		if (skipped_bytes < 0)
			return (skipped_bytes);
		if (skipped_bytes == 0) {
			archive_set_error(&a->archive,
			    ARCHIVE_ERRNO_FILE_FORMAT,
			    "Truncated 7-Zip file body");
			return (ARCHIVE_FATAL);
		}
		bytes -= (size_t)skipped_bytes;
		if (zip->pack_stream_bytes_unconsumed)
			read_consume(a);
	}
	return (skip_bytes);
}

skip_stream(struct archive_read *a, size_t skip_bytes)
{
	struct _7zip *zip = (struct _7zip *)a->format->data;
	const void *p;
	int64_t skipped_bytes;
	size_t bytes = skip_bytes;

	if (zip->folder_index == 0) {
		/*
		 * Optimization for a list mode.
		 * Avoid unnecessary decoding operations.
		 */
		zip->si.ci.folders[zip->entry->folderIndex].skipped_bytes
		    += skip_bytes;
		return (skip_bytes);
	}

	while (bytes) {
		skipped_bytes = read_stream(a, &p, bytes, 0);
		if (skipped_bytes < 0)
			return (skipped_bytes);
		if (skipped_bytes == 0) {
			archive_set_error(&a->archive,
			    ARCHIVE_ERRNO_FILE_FORMAT,
			    "Truncated 7-Zip file body");
			return (ARCHIVE_FATAL);
		}
		bytes -= (size_t)skipped_bytes;
		if (zip->pack_stream_bytes_unconsumed)
			read_consume(a);
	}
	return (skip_bytes);
}
