Update _grib2io.py

When looking for the GRIB2 header string "GRIB", changed the
struct.unpack format from ">i" to ">I", an unsigned integer. The
integer representation of "GRIB" should never be negative.

This fixes cases where other data could be in the file and a given
4 bytes represented as ">i" could be negative which raises an error.
Instead, we just want to skip ahead and continue looking for "GRIB".

src/grib2io/_grib2io.py

@@ -233,7 +233,7 @@ def _build_index(self):
                 # loop when "test_offset" is 0.
                 for test_offset in range(2048):
                     self._filehandle.seek(pos + test_offset)
-                    header = struct.unpack(">i", self._filehandle.read(4))[0]
+                    header = struct.unpack(">I", self._filehandle.read(4))[0]
                     if header.to_bytes(4, "big") == b"GRIB":
                         pos = pos + test_offset
                         break