Fix DataExtractor::PeekData for zero length peeks

[labath]

Summary

The function was returning the null pointer for peeks of size zero, which seems like a sensible
thing to do, but is actually pretty easy to get bitten by that if you are extracting a variable
length field which happens to be of zero length and then doing pointer arithmetic on that (which
SymbolFileDWARF does, and ended up crashing in case of empty DW_AT_location).

This changes the function to return a null pointer only when it gets queried for data which is
outside of the range of the extractor, which is more c++-y, as one can still do reasonable things
with pointers to data of size zero (think, end() iterators).

I also add a test and fix some signedness warnings in the existing data extractor tests.

[clayborg]

Looks good.

lldb/trunk/include/lldb/Core/DataExtractor.h

     /// @return
     ///     A non-nullptr data pointer if \a offset is a valid offset and
     ///     there are \a length bytes available at that offset, nullptr
     ///     otherwise.
     //------------------------------------------------------------------
     const uint8_t*
     PeekData (lldb::offset_t offset, lldb::offset_t length) const
     {
-        if (length > 0 && ValidOffsetForDataOfSize(offset, length))
+        if (ValidOffsetForDataOfSize(offset, length))
             return m_start + offset;
         return nullptr;
     }
 
     //------------------------------------------------------------------
     /// Set the address byte size.
     ///
     /// Set the size in bytes that will be used when extracting any

lldb/trunk/unittests/Core/DataExtractorTest.cpp

 #include "gtest/gtest.h"
 
 #include "lldb/Core/DataExtractor.h"
 
 using namespace lldb_private;
 
 TEST(DataExtractorTest, GetBitfield)
 {
-    char buffer[] = { 0x01, 0x23, 0x45, 0x67 };
+    uint8_t buffer[] = { 0x01, 0x23, 0x45, 0x67 };
     DataExtractor LE(buffer, sizeof(buffer), lldb::eByteOrderLittle, sizeof(void *));
     DataExtractor BE(buffer, sizeof(buffer), lldb::eByteOrderBig, sizeof(void *));
 
     lldb::offset_t offset;
 
     offset = 0;
     ASSERT_EQ(buffer[1], LE.GetMaxU64Bitfield(&offset, sizeof(buffer), 8, 8));
     offset = 0;
     ASSERT_EQ(buffer[1], BE.GetMaxU64Bitfield(&offset, sizeof(buffer), 8, 8));
 
     offset = 0;
-    ASSERT_EQ(buffer[1], LE.GetMaxS64Bitfield(&offset, sizeof(buffer), 8, 8));
+    ASSERT_EQ(int8_t(buffer[1]), LE.GetMaxS64Bitfield(&offset, sizeof(buffer), 8, 8));
     offset = 0;
-    ASSERT_EQ(buffer[1], BE.GetMaxS64Bitfield(&offset, sizeof(buffer), 8, 8));
+    ASSERT_EQ(int8_t(buffer[1]), BE.GetMaxS64Bitfield(&offset, sizeof(buffer), 8, 8));
+}
+
+TEST(DataExtractorTest, PeekData)
+{
+    uint8_t buffer[] = { 0x01, 0x02, 0x03, 0x04 };
+    DataExtractor E(buffer, sizeof buffer, lldb::eByteOrderLittle, 4);
+
+    EXPECT_EQ(buffer + 0, E.PeekData(0, 0));
+    EXPECT_EQ(buffer + 0, E.PeekData(0, 4));
+    EXPECT_EQ(nullptr, E.PeekData(0, 5));
+
+    EXPECT_EQ(buffer + 2, E.PeekData(2, 0));
+    EXPECT_EQ(buffer + 2, E.PeekData(2, 2));
+    EXPECT_EQ(nullptr, E.PeekData(2, 3));
+
+    EXPECT_EQ(buffer + 4, E.PeekData(4, 0));
+    EXPECT_EQ(nullptr, E.PeekData(4, 1));
 }