CMake: Compile with -ftrivial-auto-var-init=zero

As per upstream guidance [0] we should compile the sources with this
flag in order to not hit issues with using uninitialized variables.

[0] https://android-review.googlesource.com/c/platform/system/core/+/2963911/1#message-01ebff378bb51f7815b6ed8b035a57fbce0418ab

Fixes nmeum#133

vendor/CMakeLists.txt

@@ -16,6 +16,9 @@ if(APPLE)
 	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_DARWIN_C_SOURCE -D__DARWIN_C_LEVEL=__DARWIN_C_FULL")
 endif()
 
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ftrivial-auto-var-init=zero")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ftrivial-auto-var-init=zero")
+
 # Android seems to use various attributes supported by clang but not by
 # GCC which causes it to emit lots of warnings. Since these attributes
 # don't seem to effect runtime behaviour simply disable the warnings.