BUG/CRITICAL: mux-h2: re-check the frame length when PRIORITY is used

Tim D�sterhus reported a possible crash in the H2 HEADERS frame decoder
when the PRIORITY flag is present. A check is missing to ensure the 5
extra bytes needed with this flag are actually part of the frame. As per
RFC7540#4.2, let's return a connection error with code FRAME_SIZE_ERROR.

Many thanks to Tim for responsibly reporting this issue with a working
config and reproducer. This issue was assigned CVE-2018-20615.

This fix must be backported to 1.9 and 1.8.

src/mux_h2.c

@@ -3316,6 +3316,11 @@ next_frame:
 			goto fail;
 		}
 
+		if (flen < 5) {
+			h2c_error(h2c, H2_ERR_FRAME_SIZE_ERROR);
+			goto fail;
+		}
+
 		hdrs += 5; // stream dep = 4, weight = 1
 		flen -= 5;
 	}