[libc][__support/memory_size] fix missing branch and add related tests (#83016)

fix #82644.

Author: SchrodingerZhu

libc/src/__support/memory_size.h

@@ -52,9 +52,11 @@ class SafeMemSize {
 
   LIBC_INLINE SafeMemSize operator+(const SafeMemSize &other) {
     type result;
-    if (LIBC_UNLIKELY((value | other.value) < 0))
+    if (LIBC_UNLIKELY((value | other.value) < 0)) {
       result = -1;
-    result = value + other.value;
+    } else {
+      result = value + other.value;
+    }
     return SafeMemSize{result};
   }
 

libc/test/src/__support/memory_size_test.cpp

@@ -49,6 +49,13 @@ TEST(LlvmLibcMemSizeTest, Addition) {
   ASSERT_FALSE((max + SafeMemSize{static_cast<size_t>(1)}).valid());
   ASSERT_FALSE((third + third + third + third).valid());
   ASSERT_FALSE((half + half + half).valid());
+
+  ASSERT_FALSE((SafeMemSize{static_cast<size_t>(-1)} +
+                SafeMemSize{static_cast<size_t>(2)})
+                   .valid());
+  ASSERT_FALSE((SafeMemSize{static_cast<size_t>(2)} +
+                SafeMemSize{static_cast<size_t>(-1)})
+                   .valid());
 }
 
 TEST(LlvmLibcMemSizeTest, Multiplication) {