Fix invalid returned opcode for memoized expressions

Closes GH-12345

NEWS

@@ -4,6 +4,7 @@ PHP                                                                        NEWS
 
 - Core:
   . Fixed bug #80092 (ZTS + preload = segfault on shutdown). (nielsdos)
+  . Fixed buffer underflow when compiling memoized expression. (ilutov)
 
 - CType:
   . Fixed bug GH-11997 (ctype_alnum 5 times slower in PHP 8.1 or greater).

Zend/tests/assign_coalesce_009.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Invalid opcode returned from zend_compile_var_inner() for memoized expression
+--FILE--
+<?php
+strlen("foo")[0] ??= 123;
+?>
+--EXPECTF--
+Fatal error: Cannot use result of built-in function in write context in %s on line %d

Zend/zend_compile.c

@@ -10616,7 +10616,8 @@ static zend_op *zend_compile_var_inner(znode *result, zend_ast *ast, uint32_t ty
 			case ZEND_AST_NULLSAFE_METHOD_CALL:
 			case ZEND_AST_STATIC_CALL:
 				zend_compile_memoized_expr(result, ast);
-				return &CG(active_op_array)->opcodes[CG(active_op_array)->last - 1];
+				/* This might not actually produce an opcode, e.g. for expressions evaluated at comptime. */
+				return NULL;
 		}
 	}