Fix -Werror=return-type warnings to prevent gcc-8+ from
corrupting caller's stack.

Also detected by -fsanitize=undefined as:
runtime error: execution reached the end of a value-returning
function without returning a value
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -103,5 +103,6 @@ float InputHandler::bindingState(int key)
   }
   else
     return 0.0;
+  return 0.0;
 }
 
--- a/src/objects.h
+++ b/src/objects.h
@@ -545,12 +545,12 @@ class Object : public LevelObject
     bool Augmented() { return augmented; }
     void Augment() { model[1].set(MDL_PLAYER_TORSO2); augmented = true; }
 
-    virtual Weapon* Wpn() {}
-    virtual int CurrWeapon() {}
+    virtual Weapon* Wpn() { return 0; }
+    virtual int CurrWeapon() { return 0; }
     virtual void selectWeapon(int wp) {}
-    virtual char weaponState(int wp) {}
-    virtual char keyState(int wp) {}
-    virtual Vector2D* WeaponPoint() {}
+    virtual char weaponState(int wp) { return 0; }
+    virtual char keyState(int wp) { return 0; }
+    virtual Vector2D* WeaponPoint() { return 0; }
     
     void giveKey(int key) { keys |= 1 << key-1; }
     virtual void givePowerup(int idx) {}
--- a/src/sdb.h
+++ b/src/sdb.h
@@ -370,7 +370,7 @@ class Vector2D
     void set(float nx, float ny) { c[X] = nx; c[Y] = ny; c[Z] = 0; }
     void apply() { glVertex3fv(c); }
     void print() { printf("(%f, %f)\n", c[X], c[Y]); }
-    Vector2D operator = (Vector2D v) { c[X] = v.c[X]; c[Y] = v.c[Y]; }
+    Vector2D operator = (Vector2D v) { c[X] = v.c[X]; c[Y] = v.c[Y]; return *this; }
     void operator += (Vector2D v) { c[X] += v.c[X]; c[Y] += v.c[Y]; }
     void operator -= (Vector2D v) { c[X] -= v.c[X]; c[Y] -= v.c[Y]; }
     void operator += (float s) { c[X] += s; c[Y] += s; }
--- a/src/weapons.cpp
+++ b/src/weapons.cpp
@@ -135,6 +135,7 @@ bool Weapon::fire(float x, float y, float head, float h)
   }
   else
     return false;
+  return false;
 }
 
 void Weapon::releaseTrigger(float x, float y, float head, float h)