fix(PIBitArray): guard pop_back() against empty array

pop_back() called resize(size_ - 1) without checking for empty.
On empty array, uint underflow produced UINT_MAX, causing
bytesInBits(UINT_MAX) overflow and subsequent OOB access.
pop_front() already had this guard; pop_back() was missing it.

libs/main/types/pibitarray.h

@@ -5,7 +5,7 @@
 //! \~russian Упакованный массив битов
 /*
     PIP - Platform Independent Primitives
-	Packed bit array
+    Packed bit array
     Ivan Pelipenko peri4ko@yandex.ru
 
     This program is free software: you can redistribute it and/or modify
@@ -173,7 +173,10 @@ public:
 
 	//! \~english Remove one bit from the end of array.
 	//! \~russian Удаляет один бит с конца массива.
-	PIBitArray & pop_back() { return resize(size_ - 1); }
+	PIBitArray & pop_back() {
+		if (size_ == 0) return *this;
+		return resize(size_ - 1);
+	}
 
 	//! \~english Remove one bit from the beginning of array.
 	//! \~russian Удаляет один бит с начала массива.