LibCrypto: Parse negative input correctly in BigFraction::from_string()

Previously, when calling `BigFraction::from_string()`, the fractional
part of the number was always treated as positive. This led to an
incorrect result if the input string was negative.

Tests/LibCrypto/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(TEST_SOURCES
     TestAES.cpp
     TestASN1.cpp
+    TestBigFraction.cpp
     TestBigInteger.cpp
     TestChecksum.cpp
     TestChaCha20.cpp