[Backtracing] Add check for theoretical environment overflow issue.

Pretty sure this can't ever trigger because of operating system
limits on the size of environment strings, but since overflow
tests are cheap, there seems no reason not to add one.

rdar://160307933

Author: al45tair

stdlib/public/runtime/Backtrace.cpp

@@ -902,7 +902,11 @@ _swift_backtraceSetupEnvironment()
 
     size_t nameLen = std::strlen(name);
     size_t valueLen = std::strlen(value);
-    size_t totalLen = nameLen + 1 + valueLen + 1;
+    size_t totalLen;
+
+    if (__builtin_add_overflow(nameLen + 1, valueLen + 1, &totalLen)) {
+      break;
+    }
 
     if (remaining > totalLen) {
       std::memcpy(penv, name, nameLen);