Implement well-defined handling of unrecoverable errors

Plus the addition of some convenience CRASH_* error macros.
Plus transient avoidance of the flood of warnings emitted by Clang when checking 'this' for NULL.
Plus explanation about the do-while(0) loop in some error macros.

core/vector.h

@@ -134,10 +134,7 @@ public:
 
 	inline T &operator[](int p_index) {
 
-		if (p_index < 0 || p_index >= size()) {
-			T &aux = *((T *)0); //nullreturn
-			ERR_FAIL_COND_V(p_index < 0 || p_index >= size(), aux);
-		}
+		CRASH_BAD_INDEX(p_index, size());
 
 		_copy_on_write(); // wants to write, so copy on write.
 
@@ -146,10 +143,8 @@ public:
 
 	inline const T &operator[](int p_index) const {
 
-		if (p_index < 0 || p_index >= size()) {
-			const T &aux = *((T *)0); //nullreturn
-			ERR_FAIL_COND_V(p_index < 0 || p_index >= size(), aux);
-		}
+		CRASH_BAD_INDEX(p_index, size());
+
 		// no cow needed, since it's reading
 		return _get_data()[p_index];
 	}