Don't crash on a malformed HEIC file

[eeshsaxena]

process_file on a malformed HEIC can escape with an exception it doesn't handle, so the caller crashes instead of getting an empty result the way it does for other unreadable images:

import io, struct, exifread
def box(k, p=b""): return struct.pack(">I", 8 + len(p)) + k + p
ftyp = box(b"ftyp", b"heic" + struct.pack(">I", 0) + b"heic")

exifread.process_file(io.BytesIO(ftyp + struct.pack(">I", 0) + b"meta" + b"\x00" * 4))
# NotImplementedError            (a box with size 0)
exifread.process_file(io.BytesIO(ftyp))
# EOFError                       (a box that runs past the end of the file)

The HEIC reader raises a few things process_file doesn't catch: a bare NotImplementedError for a size-0 box, EOFError from get() at end of file, and BadSize / BoxVersion (which subclass ExifError but not InvalidExif).

This keeps everything inside exif-py's own error hierarchy and handles it in one place:

• next_box raises BadSize for a size-0 box instead of NotImplementedError.
• get() raises BadSize on an unexpected end of file instead of EOFError.
• process_file catches the base ExifError, so ExifNotFound, InvalidExif, BadSize and BoxVersion all yield an empty result.

Valid files are unaffected (the existing suite passes, 32). Added a test with several malformed HEIC inputs. This is separate from the non-ASCII box-type case in #245 (that raises UnicodeDecodeError, addressed in #253).