asan: fix crash in strdup on malloc failure

[dvyukov]

Summary

There are some programs that try to handle all malloc failures.
Let's allow testing of such programs.

compiler-rt/lib/asan/asan_interceptors.cpp

INTERCEPTOR(char*, strdup, const char *s) {
   if (UNLIKELY(!asan_inited)) return internal_strdup(s);
   ENSURE_ASAN_INITED();
   uptr length = internal_strlen(s);
   if (flags()->replace_str) {
     ASAN_READ_RANGE(ctx, s, length + 1);
   }
   GET_STACK_TRACE_MALLOC;
   void *new_mem = asan_malloc(length + 1, &stack);
+  if (new_mem) {
     REAL(memcpy)(new_mem, s, length + 1);
+  }
   return reinterpret_cast<char*>(new_mem);
 }
 
 #if ASAN_INTERCEPT___STRDUP
 INTERCEPTOR(char*, __strdup, const char *s) {
   void *ctx;
   ASAN_INTERCEPTOR_ENTER(ctx, strdup);
   if (UNLIKELY(!asan_inited)) return internal_strdup(s);
   ENSURE_ASAN_INITED();
   uptr length = internal_strlen(s);
   if (flags()->replace_str) {
     ASAN_READ_RANGE(ctx, s, length + 1);
   }
   GET_STACK_TRACE_MALLOC;
   void *new_mem = asan_malloc(length + 1, &stack);
+  if (new_mem) {
     REAL(memcpy)(new_mem, s, length + 1);
+  }
   return reinterpret_cast<char*>(new_mem);
 }
 #endif // ASAN_INTERCEPT___STRDUP
 
 INTERCEPTOR(char*, strncpy, char *to, const char *from, uptr size) {
   void *ctx;
   ASAN_INTERCEPTOR_ENTER(ctx, strncpy);
   ENSURE_ASAN_INITED();