Fix new clang compiler warning

Bug: 148287059
Test: make
Change-Id: I24972955454fa40fc565744e2caaaa636caa44c2
diff --git a/tests/fenv_test.cpp b/tests/fenv_test.cpp
index d27808f..e983a1c 100644
--- a/tests/fenv_test.cpp
+++ b/tests/fenv_test.cpp
@@ -27,7 +27,7 @@
   volatile float m = 0x1.0p23f;
   volatile float x = f + m;
   ASSERT_FLOAT_EQ(expectation1, x);
-  x -= m;
+  x = x - m;
   ASSERT_EQ(expectation2, x);
 }