Fix some UBSAN warnings.

* external/com_google_protobuf/src/google/protobuf/stubs/strutil.cc:1122:9: runtime error: negation of -9223372036854775808 cannot be represented in type 'google::protobuf::int64' (aka 'long'); cast to an unsigned type to negate this value to itself

* Bad external/com_google_protobuf/src/google/protobuf/text_format.cc:1320:14: runtime error: null pointer passed as argument 1, which is declared to never be null
  /usr/include/string.h:62:62: note: nonnull attribute specified here
Signed-off-by: Harvey Tuch <htuch@google.com>

src/google/protobuf/stubs/strutil.cc

@@ -1116,10 +1116,12 @@ char* FastUInt64ToBufferLeft(uint64 u64, char* buffer) {
 }
 
 char* FastInt64ToBufferLeft(int64 i, char* buffer) {
-  uint64 u = i;
+  uint64 u = 0;
   if (i < 0) {
     *buffer++ = '-';
-    u = -i;
+    u -= i;
+  } else {
+    u = i;
   }
   return FastUInt64ToBufferLeft(u, buffer);
 }

src/google/protobuf/text_format.cc

@@ -1315,7 +1315,9 @@ class TextFormat::Printer::TextGenerator
     while (size > buffer_size_) {
       // Data exceeds space in the buffer. Write what we can and request a new
       // buffer.
-      memset(buffer_, ' ', buffer_size_);
+      if (buffer_size_ > 0) {
+        memset(buffer_, ' ', buffer_size_);
+      }
       size -= buffer_size_;
       void* void_buffer;
       failed_ = !output_->Next(&void_buffer, &buffer_size_);