Fix ICE caused by swallowing a token in c_parser_consume_token

This patch fixes ICE on invalid code, specifically files that have
conflict-marker-like signs before EOF.

	PR c/92833
gcc/c/
	* c-parser.c (c_parser_consume_token): Fix peeked token stack pop
	to support 4 available tokens.

gcc/testsuite/
	* c-c++-common/pr92833-1.c, c-c++-common/pr92833-2.c,
	c-c++-common/pr92833-3.c, c-c++-common/pr92833-4.c: New tests.

gcc/c/ChangeLog

+2020-01-16  Kerem Kat  <keremkat@gmail.com>
+
+	PR c/92833
+	* c-parser.c (c_parser_consume_token): Fix peeked token stack pop
+	to support 4 available tokens.
+
 2020-01-15  Joseph Myers  <joseph@codesourcery.com>
 
 	PR c/93072

gcc/c/c-parser.c

@@ -846,7 +846,11 @@ c_parser_consume_token (c_parser *parser)
     {
       parser->tokens[0] = parser->tokens[1];
       if (parser->tokens_avail >= 3)
-	parser->tokens[1] = parser->tokens[2];
+        {
+          parser->tokens[1] = parser->tokens[2];
+          if (parser->tokens_avail >= 4)
+            parser->tokens[2] = parser->tokens[3];
+        }
     }
   parser->tokens_avail--;
 }

gcc/testsuite/ChangeLog

+2020-01-16  Kerem Kat  <keremkat@gmail.com>
+
+	PR c/92833
+	* c-c++-common/pr92833-1.c, c-c++-common/pr92833-2.c,
+	c-c++-common/pr92833-3.c, c-c++-common/pr92833-4.c: New tests.
+
 2020-01-16  Stam Markianos-Wright  <stam.markianos-wright@arm.com>
 
 	* g++.target/arm/bfloat_cpp_typecheck.C: New test.

gcc/testsuite/c-c++-common/pr92833-1.c

+/* Six marker characters at EOF, causes conflict marker detector to peek 4
+   tokens. */
+
+/* { dg-error "expected" } */ <<<<<<
\ No newline at end of file

gcc/testsuite/c-c++-common/pr92833-2.c

+/* Six marker characters at EOF, causes conflict marker detector to peek 4
+   tokens. */
+
+/* { dg-error "expected" } */ ======
\ No newline at end of file

gcc/testsuite/c-c++-common/pr92833-3.c

+/* Six marker characters at EOF, causes conflict marker detector to peek 4
+   tokens. */
+
+/* { dg-error "expected" } */ >>>>>>
\ No newline at end of file

gcc/testsuite/c-c++-common/pr92833-4.c

+/* Six marker characters at EOF, causes conflict marker detector to peek 4
+   tokens. */
+
+/* { dg-message "expected" "expected" { target *-*-* } .3 } */
+>>
+>>
+>>
\ No newline at end of file