fix: also need to update ps_loc (#1792)

Follow up to #1787, my unit test didn't fully capture the issue from the
ticket. I've verified that after this change, I can successfully load
the test file from the ticket using PyVortex.

vortex-file/src/read/builder/initial_read.rs

@@ -107,7 +107,7 @@ pub async fn read_initial_bytes<R: VortexReadAt>(
         )
     }
 
-    let ps_loc = eof_loc - ps_size;
+    let mut ps_loc = eof_loc - ps_size;
     let mut fb_postscript_byte_range = ps_loc..eof_loc;
 
     // we validate the footer here
@@ -149,6 +149,7 @@ pub async fn read_initial_bytes<R: VortexReadAt>(
         initial_read_offset = schema_offset;
         fb_postscript_byte_range.start += prefix_bytes;
         fb_postscript_byte_range.end += prefix_bytes;
+        ps_loc += prefix_bytes;
     }
 
     // validate the schema and layout