FreeBSD: Skip unused ELF section headers

ELF section headers are allowed to be left uninitialized when the
section is empty and unused. LLD is a tad more aggressive about this.
The ELF reader in the Swift runtime was a bit aggressive about
converting the section headers to names and would not skip over these
unused sections headers resulting in crashes due to operating on
uninitialized memory in the `sh_name` field.

This patch teaches the ELF reader to skip over unused section header
table entries.

Author: etcwilde

include/swift/RemoteInspection/ReflectionContext.h

@@ -635,6 +635,9 @@ class ReflectionContext
             return {nullptr, 0};
           // Now for all the sections, find their name.
           for (const typename T::Section *Hdr : SecHdrVec) {
+            // Skip unused headers
+            if (Hdr->sh_type == llvm::ELF::SHT_NULL)
+              continue;
             uint32_t Offset = Hdr->sh_name;
             const char *Start = (const char *)StrTab + Offset;
             uint64_t StringSize = strnlen(Start, StrTabSize - Offset);