added case for missing input of file to main.c

src/main.c

@@ -38,8 +38,16 @@ int main(void) {
     
     FILE* input = fopen("program.gem", "r");
 
+    if (input == NULL)
+    {
+        printf("Error opening file!\n");
+        return 1; // Error when opening file
+    }
+    
     yyin = input;
 
+    fclose(input);
+
     yyparse();
     return 0;
 }