Fix negative valued enums deserializing for protobuf

[Dogacel]

From language guide:

Enumerator constants must be in the range of a 32-bit integer. Since enum values use varint encoding on the wire, negative values are inefficient and thus not recommended

Previously, I had the following error when trying to deserialize:

java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 4
	at kotlinx.serialization.internal.PluginGeneratedSerialDescriptor.getElementAnnotations(PluginGeneratedSerialDescriptor.kt:137)
	at kotlinx.serialization.protobuf.internal.HelpersKt.extractProtoId(Helpers.kt:74)
	at kotlinx.serialization.protobuf.internal.ProtobufDecoder.findIndexByTag(ProtobufDecoding.kt:84)
	at kotlinx.serialization.protobuf.internal.ProtobufDecoder.decodeTaggedEnum(ProtobufDecoding.kt:183)
	at kotlinx.serialization.protobuf.internal.ProtobufTaggedDecoder.decodeEnum(ProtobufTaggedDecoder.kt:40)

Added a test case to verify that solution works.

[sandwwraith]

Thanks!

Note: It seems it will also make possible to use negative numbers for regular proto fields (which is not allowed per spec), but we can always prohibit it later.