file_path: avoid fortified realpath() buffer size abort (#19037)

glibc's fortified realpath() checks that the destination buffer is at
least PATH_MAX bytes. path_resolve_realpath() was passing the caller's
buffer directly, which may be smaller than the system PATH_MAX despite
being sized to RetroArch's PATH_MAX_LENGTH.

With _FORTIFY_SOURCE=3 this can abort in __realpath_chk when resolving
core updater paths.

Resolve into an allocated realpath() buffer instead, then copy the result
back into the caller-provided buffer using the provided length.

Author: s3gfaultx

libretro-common/file/file_path.c

@@ -793,12 +793,16 @@ char *path_resolve_realpath(char *s, size_t len, bool resolve_symlinks)
    const char *buf_end;
    if (resolve_symlinks)
    {
+      char *real_path;
       strlcpy(tmp, s, sizeof(tmp));
-      if (!realpath(tmp, s))
+      real_path = realpath(tmp, NULL);
+      if (!real_path)
       {
          strlcpy(s, tmp, len);
          return NULL;
       }
+      strlcpy(s, real_path, len);
+      free(real_path);
       return s;
    }
    t       = 0;