Fix assumed signed char

The code assumes that char is signed, but whether char is signed or
unsigned is implementation defined. On some architectures like PowerPC,
GCC treats char as unsigned resulting in compile errors:

	smtp-address-validator.cpp:213:1: error: narrowing conversion of '-32' from 'int' to 'char' [-Wnarrowing]

Fix this by specifying signed char.

src/smtp-address-validator.cpp

@@ -63,7 +63,7 @@ static const short _address_key_offsets[] = {
 	1363, 1365, 1367, 1368, 1370, 1388, 0
 };
 
-static const char _address_trans_keys[] = {
+static const signed char _address_trans_keys[] = {
 	-32, -19, -16, -12, 34, 45, 61, 63,
 	-62, -33, -31, -17, -15, -13, 33, 39,
 	42, 43, 47, 57, 65, 90, 94, 126,
@@ -711,7 +711,7 @@ bool is_address(const char* p, const char* pe)
 	{
 		int _klen;
 		unsigned int _trans = 0;
-		const char * _keys;
+		const signed char * _keys;
 		const signed char * _acts;
 		unsigned int _nacts;
 		_resume: {}
@@ -728,9 +728,9 @@ bool is_address(const char* p, const char* pe)
 
 			_klen = (int)_address_single_lengths[cs];
 			if ( _klen > 0 ) {
-				const char *_lower = _keys;
-				const char *_upper = _keys + _klen - 1;
-				const char *_mid;
+				const signed char *_lower = _keys;
+				const signed char *_upper = _keys + _klen - 1;
+				const signed char *_mid;
 				while ( 1 ) {
 					if ( _upper < _lower ) {
 						_keys += _klen;
@@ -752,9 +752,9 @@ bool is_address(const char* p, const char* pe)
 
 			_klen = (int)_address_range_lengths[cs];
 			if ( _klen > 0 ) {
-				const char *_lower = _keys;
-				const char *_upper = _keys + (_klen<<1) - 2;
-				const char *_mid;
+				const signed char *_lower = _keys;
+				const signed char *_upper = _keys + (_klen<<1) - 2;
+				const signed char *_mid;
 				while ( 1 ) {
 					if ( _upper < _lower ) {
 						_trans += (unsigned int)_klen;