Fix the order of error_msg validation

Reported by @marcstern at #2128

Author: Felipe Zimmerle

CHANGES

@@ -1,6 +1,8 @@
 DD mmm YYYY - 2.9.x (to be released)
 ------------------------------------
 
+ * Fix the order of error_msg validation
+   [Issue #2128 - @marcstern, @zimmerle]
  * Added missing Geo Countries
    [Issue #2123, #2124 - @emphazer]
  * When the input filter finishes, check whether we returned data

apache2/re_operators.c

@@ -1993,6 +1993,9 @@ static int msre_op_within_execute(modsec_rec *msr, msre_rule *rule, msre_var *va
     unsigned int target_length = 0;
     unsigned int i, i_max;
 
+    if (error_msg == NULL) return -1;
+    *error_msg = NULL;
+
     str->value = (char *)rule->op_param;
 
     if (str->value == NULL) {
@@ -2002,9 +2005,6 @@ static int msre_op_within_execute(modsec_rec *msr, msre_rule *rule, msre_var *va
 
     str->value_len = strlen(str->value);
 
-    if (error_msg == NULL) return -1;
-    *error_msg = NULL;
-
     expand_macros(msr, str, rule, msr->mp);
 
     match = (const char *)str->value;