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

@@ -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.