[libunwind] Fix UB in EHHeaderParser::findFDE

When the EHHeaderInfo object filled by decodeEHHdr has fde_count == 0,
findFDE does the following:

- sets low = 0 and len = hdrInfo.fde_count as a preparation to start a
  binary search
- because len is 0, the binary search loop is skipped
- the code still tries to find a table entry at
  hdrInfo.table + low * tableEntrySize, and decode it.

This is wrong when fde_count is 0, and trying to decode a table entry
that isn't there will lead to reading garbage offsets and can cause
segfaults.

Differential Revision: https://reviews.llvm.org/D77679

libunwind/src/EHHeaderParser.hpp

@@ -109,6 +109,8 @@ bool EHHeaderParser<A>::findFDE(A &addressSpace, pint_t pc, pint_t ehHdrStart,
                                       hdrInfo))
     return false;
 
+  if (hdrInfo.fde_count == 0) return false;
+
   size_t tableEntrySize = getTableEntrySize(hdrInfo.table_enc);
   pint_t tableEntry;