AP_Param: avoid allocating 0 bytes if no defaults

This stopped CUAVv5 from booting.

Also add some protection against having different parsers for the same
file format used for counting parameters and actually using them.

libraries/AP_Param/AP_Param.cpp

@@ -1938,17 +1938,19 @@ bool AP_Param::load_defaults_file(const char *filename, bool last_pass)
     }
     free(mutable_filename);
 
-    if (param_overrides != nullptr) {
-        free(param_overrides);
-    }
+    delete[] param_overrides;
     num_param_overrides = 0;
 
-    param_overrides = (struct param_override *)malloc(sizeof(struct param_override)*num_defaults);
+    param_overrides = new param_override[num_defaults];
     if (param_overrides == nullptr) {
         AP_HAL::panic("AP_Param: Failed to allocate overrides");
         return false;
     }
 
+    if (num_defaults == 0) {
+        return true;
+    }
+
     saveptr = nullptr;
     mutable_filename = strdup(filename);
     if (mutable_filename == nullptr) {
@@ -2027,28 +2029,26 @@ bool AP_Param::count_embedded_param_defaults(uint16_t &count)
  */
 void AP_Param::load_embedded_param_defaults(bool last_pass)
 {
-    if (param_overrides != nullptr) {
-        free(param_overrides);
-        param_overrides = nullptr;
-    }
-    
+    delete[] param_overrides;
+    param_overrides = nullptr;
     num_param_overrides = 0;
+
     uint16_t num_defaults = 0;
     if (!count_embedded_param_defaults(num_defaults)) {
         return;
     }
 
-    param_overrides = (struct param_override *)malloc(sizeof(struct param_override)*num_defaults);
+    param_overrides = new param_override[num_defaults];
     if (param_overrides == nullptr) {
         AP_HAL::panic("AP_Param: Failed to allocate overrides");
         return;
     }
-    
+
     const volatile char *ptr = param_defaults_data.data;
     uint16_t length = param_defaults_data.length;
     uint16_t idx = 0;
     
-    while (length) {
+    while (idx < num_defaults && length) {
         char line[100];
         char *pname;
         float value;