rtlil: handle all-zeros case in Const::compress

kernel/rtlil.cc

@@ -293,13 +293,15 @@ void RTLIL::Const::compress(bool is_signed)
 
 	size_t idx = bits.size();
 	while (idx > 0 && bits[idx -1] == leading_bit) {
-        --idx;
+		idx--;
 	}
 
 	// signed needs one leading bit
 	if (is_signed && idx < bits.size()) {
-        ++idx;
+		idx++;
 	}
+	// must be at least one bit
+	idx = (idx == 0) ? 1 : idx;
 
 	bits.erase(bits.begin() + idx, bits.end());
 }