Update for _grib2io.py

Added an else to the for loop on checking for "GRIB" to continue
when "GRIB" is not found in that loop. This will allow for grib2io
to silently ignore data that are not GRIB2 message.

src/grib2io/_grib2io.py

@@ -237,6 +237,11 @@ def _build_index(self):
                     if header.to_bytes(4, "big") == b"GRIB":
                         pos = pos + test_offset
                         break
+                else:
+                    # NOTE: Coming here means that no "GRIB" message identifier
+                    # was found in the previous 2048 bytes. So here we continue
+                    # the while True loop.
+                    continue
 
                 # Read the rest of Section 0 using struct.
                 _secpos[0] = self._filehandle.tell()-4