Fix message for some string offset uses as object

Even if the object property is incremented afterwards, the
immediate error is the use as object, not the increment.

Also consolidate tests for this error message. Previously they
were spread across a number of bug-specific tests.

Author: nikic

Zend/tests/bug41813.phpt

@@ -1562,6 +1562,11 @@ ZEND_API ZEND_COLD void zend_wrong_string_offset_error(void)
 					case ZEND_ASSIGN_OBJ:
 					case ZEND_ASSIGN_OBJ_OP:
 					case ZEND_ASSIGN_OBJ_REF:
+					case ZEND_PRE_INC_OBJ:
+					case ZEND_PRE_DEC_OBJ:
+					case ZEND_POST_INC_OBJ:
+					case ZEND_POST_DEC_OBJ:
+					case ZEND_UNSET_OBJ:
 						msg = "Cannot use string offset as an object";
 						break;
 					case ZEND_FETCH_DIM_W:
@@ -1577,10 +1582,6 @@ ZEND_API ZEND_COLD void zend_wrong_string_offset_error(void)
 					case ZEND_ASSIGN_OP:
 						msg = "Cannot use assign-op operators with string offsets";
 						break;
-					case ZEND_PRE_INC_OBJ:
-					case ZEND_PRE_DEC_OBJ:
-					case ZEND_POST_INC_OBJ:
-					case ZEND_POST_DEC_OBJ:
 					case ZEND_PRE_INC:
 					case ZEND_PRE_DEC:
 					case ZEND_POST_INC:
@@ -1598,7 +1599,6 @@ ZEND_API ZEND_COLD void zend_wrong_string_offset_error(void)
 						msg = "Cannot return string offsets by reference";
 						break;
 					case ZEND_UNSET_DIM:
-					case ZEND_UNSET_OBJ:
 						msg = "Cannot unset string offsets";
 						break;
 					case ZEND_YIELD: