[NFC] Remove redundant branch

[vitalybuka]

Summary

memset already covers the case.

compiler-rt/lib/asan/asan_poisoning.cpp

if (beg.chunk == end.chunk) {
     // We unpoison memory bytes up to enbytes up to end.offset if it is not
     // unpoisoned already.
     if (value != 0) {
       *beg.chunk = Max(value, end.offset);
     }
     return;
   }
   CHECK_LT(beg.chunk, end.chunk);
-  if (beg.offset > 0) {
-    *beg.chunk = 0;
-    beg.chunk++;
-  }
   REAL(memset)(beg.chunk, 0, end.chunk - beg.chunk);
   if (end.offset > 0 && end.value != 0) {
     *end.chunk = Max(end.value, end.offset);
   }
 }
 
 int __asan_address_is_poisoned(void const volatile *addr) {
   return __asan::AddressIsPoisoned((uptr)addr);