avoid snprintf in Elf code

Summary: `snprintf` may not be async-signal-safe, but Elf code is intended for use in signal handlers.

Reviewed By: luciang

Differential Revision: D27000080

fbshipit-source-id: 818f48787b5712b7334eca782e18c71c17d03361

folly/experimental/symbolizer/Elf.h

@@ -272,20 +272,16 @@ class ElfFile {
   template <class T>
   const T& at(ElfOff offset) const noexcept {
     static_assert(std::is_pod<T>::value, "non-pod");
-    if (offset + sizeof(T) > length_) {
-      char msg[kFilepathMaxLen + 128];
-      snprintf(
-          msg,
-          sizeof(msg),
-          "Offset (%zu + %zu) is not contained within our mmapped"
-          " file (%s) of length %zu",
+    FOLLY_SAFE_CHECK(
+        offset + sizeof(T) <= length_,
+        "Offset (",
         static_cast<size_t>(offset),
+        " + ",
         sizeof(T),
+        ") is not contained within our mapped file (",
         filepath_,
+        ") of length ",
         length_);
-      FOLLY_SAFE_CHECK(offset + sizeof(T) <= length_, msg);
-    }
-
     return *reinterpret_cast<T*>(file_ + offset);
   }