[PRISM] Ensure not opening directories

Merged: https://github.com/ruby/ruby/pull/11196

prism/util/pm_string.c

@@ -116,6 +116,13 @@ pm_string_mapped_init(pm_string_t *string, const char *filepath) {
         return false;
     }
 
+    // Ensure it is a file and not a directory
+    if (S_ISDIR(sb.st_mode)) {
+        errno = EISDIR;
+        close(fd);
+        return false;
+    }
+
     // mmap the file descriptor to virtually get the contents
     size_t size = (size_t) sb.st_size;
     uint8_t *source = NULL;

prism/util/pm_string.h

@@ -9,6 +9,7 @@
 #include "prism/defines.h"
 
 #include <assert.h>
+#include <errno.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>