Fix some warnings from clang's static code analyzer

src/mon.c

@@ -391,6 +391,8 @@ unsigned corpseflags;
     }
     /* All special cases should precede the G_NOCORPSE check */
 
+    if (!obj) return NULL;
+
     /* if polymorph or undead turning has killed this monster,
        prevent the same attack beam from hitting its corpse */
     if (context.bypasses)