AP_Param: fixed a compiler warning with -Wsign-conversion

libraries/AP_Param/AP_Param.cpp

@@ -920,7 +920,7 @@ AP_Param *AP_Param::next_group(uint8_t vindex, const struct GroupInfo *group_inf
                         *ptype = AP_PARAM_FLOAT;
                     }
                     uintptr_t ofs = (uintptr_t)PGM_POINTER(&_var_info[vindex].ptr) + PGM_UINT16(&group_info[i].offset);
-                    ofs += sizeof(float)*(token->idx-1);
+                    ofs += sizeof(float)*(token->idx - 1u);
                     return (AP_Param *)ofs;
                 }
             }
@@ -948,7 +948,7 @@ AP_Param *AP_Param::next(ParamToken *token, enum ap_var_type *ptype)
         if (ptype != NULL) {
             *ptype = AP_PARAM_FLOAT;
         }
-        return (AP_Param *)(((token->idx-1)*sizeof(float))+(uintptr_t)PGM_POINTER(&_var_info[i].ptr));
+        return (AP_Param *)(((token->idx - 1u)*sizeof(float))+(uintptr_t)PGM_POINTER(&_var_info[i].ptr));
     }
 
     if (type != AP_PARAM_GROUP) {