Fix #76813: Access violation near NULL on source operand

We avoid `YYCURSOR` becoming `NULL` by initializing `YYMARKER`, and add
a default rule for `<NORMAL>` where we catch unexpected input.

We also fix the only superficially related issue regarding empty input
followed by `T_SEPARATOR` and command, which caused another segfault.

Closes GH-6464.

NEWS

@@ -24,6 +24,9 @@ PHP                                                                        NEWS
   . Fixed bug #80368 (OpenSSL extension fails to build against LibreSSL due to
     lack of OCB support). (Nikita)
 
+- Phpdbg:
+  . Fixed bug #76813 (Access violation near NULL on source operand). (cmb)
+
 - Standard:
   . Fixed bug #80366 (Return Value of zend_fstat() not Checked). (sagpant, cmb)
   . Fixed bug #80411 (References to null-serialized object break serialize()).

sapi/phpdbg/phpdbg_lexer.l

@@ -33,7 +33,7 @@ void phpdbg_init_lexer (phpdbg_param_t *stack, char *input) {
 
 	YYSETCONDITION(INITIAL);
 
-	LEX(text) = YYCURSOR = (unsigned char *) input;
+	LEX(text) = YYCURSOR = YYMARKER = (unsigned char *) input;
 	LEX(len) = strlen(input);
 }
 
@@ -165,6 +165,10 @@ INPUT       ("\\"[#"']|["]("\\\\"|"\\"["]|[^\n\000"])+["]|[']("\\"[']|"\\\\"|[^\
 	return T_ID;
 }
 
+<NORMAL>* {
+	return T_UNEXPECTED;
+}
+
 <RAW>{INPUT} {
 	phpdbg_init_param(yylval, STR_PARAM);
 	yylval->str = estrdup(yytext);

sapi/phpdbg/phpdbg_parser.y

@@ -63,11 +63,15 @@ typedef void* yyscan_t;
 %% /* Rules */
 
 input
-	: command { $$ = $1; }
-	| input T_SEPARATOR command { phpdbg_stack_separate($1.top); $$ = $3; }
+	: non_empty_input { $$ = $1; }
 	| /* empty */
 	;
 
+non_empty_input
+	: command { $$ = $1; }
+	| non_empty_input T_SEPARATOR command { phpdbg_stack_separate($1.top); $$ = $3; }
+	;
+
 command
 	: parameters { $$.top = PHPDBG_G(parser_stack)->top; }
 	| full_expression { phpdbg_stack_push(PHPDBG_G(parser_stack), &$1); $$.top = PHPDBG_G(parser_stack)->top; }

sapi/phpdbg/tests/bug76813.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Bug #76813 (Access_violation_near_NULL_on_source_operand)
+--PHPDBG--
+"#!==)===\377\377\276\242="
+#!==)===\377\377\276\242=
+--EXPECT--
+prompt> [Parse Error: syntax error, unexpected input, expecting $end]
+prompt> [Parse Error: syntax error, unexpected # (pound sign), expecting $end]
+prompt> [Parse Error: syntax error, unexpected # (pound sign), expecting $end]
+prompt>