Fix expressions which evaluates to zero treated as a null pointer constant (#1263)

Fixes -Wnon-literal-null-conversion compiler warnings.

Co-authored-by: Markus Metz <markus.metz.giswork@gmail.com>

db/db.select/main.c

@@ -276,7 +276,7 @@ void parse_command_line(int argc, char **argv)
     parms.table = table->answer;
     parms.sql = sql->answer;
     parms.fs = G_option_to_separator(fs);
-    parms.vs = '\0';
+    parms.vs = NULL;
     if (vs->answer)
 	parms.vs = G_option_to_separator(vs);
     parms.nv = nv->answer;

lib/db/dbmi_base/login.c

@@ -297,11 +297,11 @@ static int get_login(const char *driver, const char *database, const char **user
 
     G_debug(3, "db_get_login(): drv=[%s] db=[%s]", driver, database);
 
-    user[0] = '\0';
-    password[0] = '\0';
-    host[0] = '\0';
-    port[0] = '\0';
-    
+    *user = NULL;
+    *password = NULL;
+    *host = NULL;
+    *port = NULL;
+
     init_login(&login);
 
     if (read_file(&login) == -1)

vector/v.category/main.c

@@ -537,7 +537,7 @@ int main(int argc, char *argv[])
 			freps[fld]->table = G_store(Fi->table);
 		    }
 		    else {
-			freps[fld]->table = '\0';
+			freps[fld]->table = NULL;
 		    }
 		}
 
@@ -601,7 +601,7 @@ int main(int argc, char *argv[])
 			    freps[fld]->table = G_store(Fi->table);
 			}
 			else {
-			    freps[fld]->table = '\0';
+			    freps[fld]->table = NULL;
 			}
 		    }