libsepol/cil: do not allow \0 in quoted strings

Using the '\0' character in strings in a CIL policy is not expected to
happen, and makes the flex tokenizer very slow. For example when
generating a file with:

    python -c 'print("\"" + "\0"*100000 + "\"")' > policy.cil

secilc fails after 26 seconds, on my desktop computer. Increasing the
numbers of \0 makes this time increase significantly. But replacing \0
with another character makes secilc fail in only few milliseconds.

Fix this "possible denial of service" issue by forbidding \0 in strings
in CIL policies.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=36016

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/cil/src/cil_lexer.l

@@ -49,7 +49,7 @@ spec_char	[\[\]\.\@\=\/\*\-\_\$\%\+\-\!\|\&\^\:\~\`\#\{\}\'\<\>\?\,]
 symbol		({digit}|{alpha}|{spec_char})+
 white		[ \t]
 newline		[\n\r]
-qstring		\"[^"\n]*\"
+qstring		\"[^"\n\0]*\"
 hll_lm          ^;;\*
 comment		;