cmake: assert non-empty FUZZ_LDFLAGS when FUZZ=ON

The invocation of cmake with -DFUZZ=ON will produce an error:

    set_target_properties called with incorrect number of arguments.

The error is due to the fact that the FUZZ_LDFLAGS variable needs
to be defined and non-empty to be correctly supplied in the
set_target_properties() invocations that follow the introduced
assertion.

See also:
https://cmake.org/cmake/help/latest/command/set_target_properties.html

fuzz/CMakeLists.txt

@@ -18,6 +18,10 @@ set(FUZZ_LINKER_LANGUAGE "C" CACHE STRING "Linker language for fuzz harnesses")
 mark_as_advanced(FUZZ_LINKER_LANGUAGE)
 enable_language(${FUZZ_LINKER_LANGUAGE})
 
+if(NOT FUZZ_LDFLAGS)
+	message(FATAL_ERROR "Please define FUZZ_LDFLAGS")
+endif()
+
 # fuzz_cred
 add_executable(fuzz_cred fuzz_cred.c ${COMMON_SOURCES} ${COMPAT_SOURCES})
 set_target_properties(fuzz_cred PROPERTIES