AP_GPS: adjust NMEA sentence length calculation

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libraries/AP_GPS/AP_GPS_NMEA.cpp

@@ -85,6 +85,8 @@ bool AP_GPS_NMEA::_decode(char c)
 {
     bool valid_sentence = false;
 
+    _sentence_length++;
+        
     switch (c) {
     case ',': // term terminators
         _parity ^= c;
@@ -99,7 +101,6 @@ bool AP_GPS_NMEA::_decode(char c)
         ++_term_number;
         _term_offset = 0;
         _is_checksum_term = c == '*';
-        _sentence_length++;
         return valid_sentence;
 
     case '$': // sentence begin
@@ -108,7 +109,7 @@ bool AP_GPS_NMEA::_decode(char c)
         _sentence_type = _GPS_SENTENCE_OTHER;
         _is_checksum_term = false;
         _gps_data_good = false;
-        _sentence_length = 0;
+        _sentence_length = 1;
         return valid_sentence;
     }
 
@@ -118,8 +119,6 @@ bool AP_GPS_NMEA::_decode(char c)
     if (!_is_checksum_term)
         _parity ^= c;
 
-    _sentence_length++;
-    
     return valid_sentence;
 }