main.c: use NULL for pointer

fix for cgcc warning:

src/main.c:238:48: warning: Using plain integer as NULL pointer
src/main.c:246:44: warning: Using plain integer as NULL pointer
src/main.c:418:17: warning: Using plain integer as NULL pointer

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

src/main.c

@@ -235,7 +235,7 @@ static int set_option(char **options, const char *name, const char *value)
     int i, type;
     const struct mg_option *default_options = mg_get_valid_options();
 
-    for (i = 0; main_config_options[i].name != 0; i++) {
+    for (i = 0; main_config_options[i].name != NULL; i++) {
         if (0==strcmp(name, main_config_options[i].name)) {
             /* This option is evaluated by main.c, not civetweb.c - just skip it and return OK */
             return 1;
@@ -243,7 +243,7 @@ static int set_option(char **options, const char *name, const char *value)
     }
 
     type = CONFIG_TYPE_UNKNOWN;
-    for (i = 0; default_options[i].name != 0; i++) {
+    for (i = 0; default_options[i].name != NULL; i++) {
         if (!strcmp(default_options[i].name, name)) {
             type = default_options[i].type;
         }
@@ -415,7 +415,7 @@ static void init_server_name(int argc, const char *argv[])
             server_name = (char*)(argv[i+1]);
         }
     }
-    icon_name = 0;
+    icon_name = NULL;
     for (i=0; i<argc-1; i++) {
         if ((argv[i][0]=='-') && (0==strcmp(argv[i]+1, main_config_options[OPTION_ICON].name))) {
             icon_name = (char*)(argv[i+1]);