Prevent undefined behaviour modifying bionic power

As it turns out, the check to prevent integer overflow when modifying
bionic power is undefined behaviour. Whoops! Let's avoid that.

data/json/mutations/mutations.json

@@ -6437,6 +6437,16 @@
     "debug": true,
     "active": true
   },
+  {
+    "type": "mutation",
+    "id": "DEBUG_BIONIC_POWERGEN",
+    "name": "Debug Bionic Powergen",
+    "points": 99,
+    "valid": false,
+    "description": "Activate to increase power by an amount you specify (can be repeated).",
+    "debug": true,
+    "active": true
+  },
   {
     "type": "mutation",
     "id": "SQUEAMISH",

src/character.cpp

@@ -1389,13 +1389,17 @@ void Character::set_max_power_level( units::energy npower_max )
 
 void Character::mod_power_level( units::energy npower )
 {
-    int new_power = units::to_millijoule( power_level ) + units::to_millijoule( npower );
-    // An integer overflow has occurred - the sum of two positive things (power_level is always positive)
-    // cannot be negative, so if it is, we know integer overflow has occured
-    if( npower >= 0_mJ && new_power < 0 ) {
-        new_power = units::to_millijoule( max_power_level );
+    // units::energy is an int, so avoid overflow by converting it to a int64_t, then adding them
+    // If the result is greater than the max power level, set power to max
+    int64_t power = static_cast<int64_t>( units::to_millijoule( power_level ) ) +
+                    static_cast<int64_t>( units::to_millijoule( npower ) );
+    units::energy new_power;
+    if( power > units::to_millijoule( max_power_level ) ) {
+        new_power = max_power_level;
+    } else {
+        new_power = power_level + npower;
     }
-    power_level = clamp( units::from_millijoule( new_power ), 0_kJ, max_power_level );
+    power_level = clamp( new_power, 0_kJ, max_power_level );
 }
 
 void Character::mod_max_power_level( units::energy npower_max )

src/mutation.cpp

@@ -16,6 +16,7 @@
 #include "mapdata.h"
 #include "memorial_logger.h"
 #include "monster.h"
+#include "output.h"
 #include "overmapbuffer.h"
 #include "player.h"
 #include "translations.h"
@@ -53,6 +54,7 @@ static const trait_id trait_TREE_COMMUNION( "TREE_COMMUNION" );
 static const trait_id trait_ROOTS2( "ROOTS2" );
 static const trait_id trait_ROOTS3( "ROOTS3" );
 static const trait_id trait_DEBUG_BIONIC_POWER( "DEBUG_BIONIC_POWER" );
+static const trait_id trait_DEBUG_BIONIC_POWERGEN( "DEBUG_BIONIC_POWERGEN" );
 
 namespace io
 {
@@ -552,6 +554,14 @@ void Character::activate_mutation( const trait_id &mut )
         add_msg_if_player( m_good, _( "Bionic power storage increased by 100." ) );
         tdata.powered = false;
         return;
+    } else if( mut == trait_DEBUG_BIONIC_POWERGEN ) {
+        int npower;
+        if( query_int( npower, "Modify bionic power by how much?  (Values are in millijoules)" ) ) {
+            mod_power_level( units::from_millijoule( npower ) );
+            add_msg_if_player( m_good, "Bionic power increased by %dmJ.", npower );
+            tdata.powered = false;
+        }
+        return;
     } else if( !mdata.spawn_item.empty() ) {
         item tmpitem( mdata.spawn_item );
         i_add_or_drop( tmpitem );