[lua] Fix sensor max and min matching

Resolves #442

radio/src/lua/api_general.cpp

@@ -372,7 +372,8 @@ bool luaFindFieldByName(const char * name, LuaField & field, unsigned int flags)
   for (int i = 0; i < MAX_TELEMETRY_SENSORS; i++) {
     if (isTelemetryFieldAvailable(i)) {
       const char* sensorName = g_model.telemetrySensors[i].label;
-      if ((len <= TELEM_LABEL_LEN) && !strncmp(sensorName, name, len)) {
+      int len = strnlen(sensorName, TELEM_LABEL_LEN);
+      if (!strncmp(sensorName, name, len)) {
         if (name[len] == '\0') {
           field.id = MIXSRC_FIRST_TELEM + 3 * i;
           field.desc[0] = '\0';

radio/src/strhelpers.cpp

@@ -89,6 +89,16 @@ int zchar2str(char * dest, const char * src, int size)
   return size+1;
 }
 
+int strnlen(const char *src, int max_size)
+{
+  for (int i=0; i<max_size; i++) {
+    if (src[i] == '\0')
+      return i;
+  }
+
+  return max_size;
+}
+
 unsigned int effectiveLen(const char * str, unsigned int size)
 {
   while (size > 0) {

radio/src/strhelpers.h

@@ -30,6 +30,7 @@ char char2lower(char c);
 int8_t char2zchar(char c);
 void str2zchar(char *dest, const char *src, int size);
 int zchar2str(char *dest, const char *src, int size);
+int strnlen(const char *src, int max_size);
 
 char * strAppend(char * dest, const char * source, int len = 0);
 char * strAppendUnsigned(char * dest, uint32_t value, uint8_t digits = 0, uint8_t radix = 10);