Fix queryktharr() return value/behaviour. (AMReX-Codes#4186)

Up to AMReX 24.07 (at least), ParmParse::queryktharr() used to return 0
if the given k (2nd parameter) was out of range.
As of AMReX 24.10, this is not the case, and ppindex() attempts to
access found->second.m_vals out-of-bounds, resulting in
seg-fault/undefined behaviour.

This PR reverts to the original behaviour, which is now consistent with
the documentation for querykth() in AMReX_ParmParse.H. It also makes
minor corrections to the associated comments to align with this
behaviour.

Src/Base/AMReX_ParmParse.cpp

@@ -349,7 +349,7 @@ getToken (const char*& str, std::string& ostr, int& num_linefeeds)
 //
 // Return the index of the n'th occurrence of a parameter name,
 // except if n==-1, return the index of the last occurrence.
-// Return 0 if the specified occurrence does not exist.
+// Return nullptr if the specified occurrence does not exist.
 //
 std::vector<std::string> const*
 ppindex (const ParmParse::Table& table, int n, const std::string& name)
@@ -365,6 +365,9 @@ ppindex (const ParmParse::Table& table, int n, const std::string& name)
  if (n == ParmParse::LAST) {
  return &(found->second.m_vals.back());
  } else {
+ if(found->second.m_vals.size() < (std::size_t)n + 1) {
+ return nullptr;
+ }
  return &(found->second.m_vals[n]);
  }
 }
@@ -642,7 +645,7 @@ squeryval (const ParmParse::Table& table,
  int occurrence)
 {
  //
- // Get last occurrence of name in table.
+ // Get specified occurrence of name in table.
  //
  auto const* def = ppindex(table, occurrence, name);
  if ( def == nullptr )