Fix divide by zero crash loading LDB signature

If a byte compare subsignature specifies a 0-byte length then the
process may crash with a divide-by-zero exception while loading
the signature.

byte_length had validation for invalid characters, but nothing for a
zero value.  Added validation for a zero value.

Author: ragusaa

libclamav/matcher-byte-comp.c

@@ -292,7 +292,7 @@ cl_error_t cli_bcomp_addpatt(struct cli_matcher *root, const char *virname, cons
     /* parse out the byte length parameter */
     buf_end     = NULL;
     byte_length = strtol(buf_start, (char **)&buf_end, 0);
-    if (buf_end && buf_end + 1 != tokens[2]) {
+    if ((buf_end && buf_end + 1 != tokens[2]) || (0 == byte_length)) {
         cli_errmsg("cli_bcomp_addpatt: while parsing (%s#%s#%s), byte length parameter included invalid characters\n", tokens[0], tokens[1], tokens[2]);
         free(buf);
         cli_bcomp_freemeta(root, bcomp);