… mountpoints

When a request path like /footris exactly matches a mountpoint /footris/
(after stripping the trailing slash for comparison), the manage-script-name
code sets script_name_len to 9 (the original mountpoint length including the
slash) but orig_path_info_len is only 8.  The subtraction
orig_path_info_len - script_name_len = -1 wraps to 65535 when stored in the
uint16_t path_info_len field.  Subsequent code then passes iov_len=65535 to
PyUnicode_DecodeLatin1, which reads far past the end of the allocated buffer.
On s390x this read crosses into an unmapped page and causes a segfault.

Fix: when orig_path_info_len is less than script_name_len (the only case
being the exact-match-without-trailing-slash scenario described above), set
path_info to an empty string (pointing at the end of the original path_info
buffer) with path_info_len = 0.