AP_GPS: prevent bad NMEA strings from causing overruns in parser

fixes issue #961

thanks to crashpilot100 for spotting this!

libraries/AP_GPS/AP_GPS_NMEA.cpp

@@ -105,6 +105,8 @@ AP_GPS_NMEA::AP_GPS_NMEA(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDr
     _gps_data_good(false)
 {
     gps.send_blob_start(state.instance, _initialisation_blob, sizeof(_initialisation_blob));
+    // this guarantees that _term is always nul terminated
+    memset(_term, 0, sizeof(_term));
 }
 
 bool AP_GPS_NMEA::read(void)
@@ -198,19 +200,19 @@ uint32_t AP_GPS_NMEA::_parse_degrees()
     int32_t ret = 0;
 
     // scan for decimal point or end of field
-    for (p = _term; isdigit(*p); p++)
+    for (p = _term; *p && isdigit(*p); p++)
         ;
     q = _term;
 
     // convert degrees
-    while ((p - q) > 2) {
+    while ((p - q) > 2 && *q) {
         if (deg)
             deg *= 10;
         deg += DIGIT_TO_VAL(*q++);
     }
 
     // convert minutes
-    while (p > q) {
+    while (p > q && *q) {
         if (min)
             min *= 10;
         min += DIGIT_TO_VAL(*q++);
@@ -220,7 +222,7 @@ uint32_t AP_GPS_NMEA::_parse_degrees()
     if (*p == '.') {
         q = p + 1;
         float frac_scale = 0.1f;
-        while (isdigit(*q)) {
+        while (*q && isdigit(*q)) {
             frac_min += (*q++ - '0') * frac_scale;
             frac_scale *= 0.1f;
         }