fdsan: reset va_list between varargs traversals

When error_level != ANDROID_FDSAN_ERROR_LEVEL_FATAL, fdsan_error()
traverses the varargs twice.  Each traversal needs its own
va_start()/va_end() pair.  Otherwise async_safe_format_buffer_va_list()
will start consuming arguments where async_safe_format_log_va_list()
stopped, ironically causing the sanitizer runtime to perform bad memory
accesses.

Test: debuggerd_test32
Test: debuggerd_test64

Change-Id: Ic85ca94501e55e9a435410fab89edb273c381a5a
Signed-off-by: Greg Hackmann <ghackmann@google.com>