lines: add null check after opening infile

c_programming/files/lines.c

@@ -21,6 +21,11 @@
 int main(void)
 {
     FILE* infile = fopen("lines.txt", "r");
+    if (infile == NULL)
+    {
+        perror("Failed to open in.txt");
+        return 1;
+    }
     char buf[SIZE];
 
     while (fgets(buf, sizeof buf, infile) != NULL)