Remove wchar.c Asserts that were stricter than the main code

Assert errors were thrown for functions being passed invalid encodings,
while the main code handled it just fine.

Also document that libpq's PQclientEncoding() returns -1 for an encoding
lookup failure.

Per report from Peter Geoghegan

doc/src/sgml/libpq.sgml

@@ -5377,7 +5377,8 @@ int PQclientEncoding(const PGconn *<replaceable>conn</replaceable>);
 </synopsis>
 
       Note that it returns the encoding ID, not a symbolic string
-      such as <literal>EUC_JP</literal>. To convert an encoding ID to an encoding name, you
+      such as <literal>EUC_JP</literal>. If unsuccessful, it returns -1.
+      To convert an encoding ID to an encoding name, you
       can use:
 
 <synopsis>

src/backend/utils/mb/wchar.c

@@ -1778,10 +1778,7 @@ pg_mic_mblen(const unsigned char *mbstr)
 int
 pg_encoding_mblen(int encoding, const char *mbstr)
 {
-	Assert(PG_VALID_ENCODING(encoding));
-
-	return ((encoding >= 0 &&
-			 encoding < sizeof(pg_wchar_table) / sizeof(pg_wchar_tbl)) ?
+	return (PG_VALID_ENCODING(encoding) ?
 		((*pg_wchar_table[encoding].mblen) ((const unsigned char *) mbstr)) :
 	((*pg_wchar_table[PG_SQL_ASCII].mblen) ((const unsigned char *) mbstr)));
 }
@@ -1792,10 +1789,7 @@ pg_encoding_mblen(int encoding, const char *mbstr)
 int
 pg_encoding_dsplen(int encoding, const char *mbstr)
 {
-	Assert(PG_VALID_ENCODING(encoding));
-
-	return ((encoding >= 0 &&
-			 encoding < sizeof(pg_wchar_table) / sizeof(pg_wchar_tbl)) ?
+	return (PG_VALID_ENCODING(encoding) ?
 	   ((*pg_wchar_table[encoding].dsplen) ((const unsigned char *) mbstr)) :
 	((*pg_wchar_table[PG_SQL_ASCII].dsplen) ((const unsigned char *) mbstr)));
 }
@@ -1808,10 +1802,7 @@ pg_encoding_dsplen(int encoding, const char *mbstr)
 int
 pg_encoding_verifymb(int encoding, const char *mbstr, int len)
 {
-	Assert(PG_VALID_ENCODING(encoding));
-
-	return ((encoding >= 0 &&
-			 encoding < sizeof(pg_wchar_table) / sizeof(pg_wchar_tbl)) ?
+	return (PG_VALID_ENCODING(encoding) ?
 		((*pg_wchar_table[encoding].mbverify) ((const unsigned char *) mbstr, len)) :
 	((*pg_wchar_table[PG_SQL_ASCII].mbverify) ((const unsigned char *) mbstr, len)));
 }