getenv: treat a read error like eof

Otherwise, the following would occur:
 * settings.toml is in the process of being written by host computer
 * soft-reset begins
 * web workflow tries to grab CIRCUITPY_WIFI_SSID, but loops forever
   because FAT filesystem is in inconsistent state and file reads error
 * settings.toml write by host computer never completes and the filesystem
   remains corrupt
 * restarting yields a soft-bricked device, because startup reads
   CIRCUITPY_WIFI_SSID again

Author: jepler

shared-module/os/getenv.c

@@ -72,7 +72,7 @@ STATIC void close_file(file_arg *active_file) {
     // nothing
 }
 STATIC bool is_eof(file_arg *active_file) {
-    return f_eof(active_file);
+    return f_eof(active_file) | f_error(active_file);
 }
 
 // Return 0 if there is no next character (EOF).