Fabmaster: catch illegal parses up front

Parsing an empty string as double/int produces uninitialised data.
That is likely to cause an undefined amout of chaos later on,
so catch it explicitly with a wxCHECK.

It's still something that shouldn't happen, and could still cause
chaos, but it's at least deterministic. It also avoids putting
gigantic numbers into things that don't expect them. For example
normalising an angle of 1e303 is functionally an infinite loop.

This happens at least in files with missing fields in rows,
like in #19174's file, line 4:

S!U2306!IC!!!!!!SOT_23_5!!!!!SGM2576/TT9107!!!

The file is still broken, but now it loads.

Relates-To: https://gitlab.com/kicad/code/kicad/-/issues/19174

pcbnew/pcb_io/fabmaster/import_fabmaster.cpp

@@ -86,6 +86,9 @@ void FABMASTER::checkpoint()
 
 double FABMASTER::readDouble( const std::string& aStr ) const
 {
+    // This is bad, but at least don't return uninitialized data
+    wxCHECK_MSG( !aStr.empty(), 0.0, "Empty string passed to readDouble" );
+
     std::istringstream istr( aStr );
     istr.imbue( std::locale::classic() );
 
@@ -97,6 +100,9 @@ double FABMASTER::readDouble( const std::string& aStr ) const
 
 int FABMASTER::readInt( const std::string& aStr ) const
 {
+    // This is bad, but at least don't return uninitialized data
+    wxCHECK_MSG( !aStr.empty(), 0, "Empty string passed to readInt" );
+
     std::istringstream istr( aStr );
     istr.imbue( std::locale::classic() );
 
@@ -108,7 +114,6 @@ int FABMASTER::readInt( const std::string& aStr ) const
 
 bool FABMASTER::Read( const std::string& aFile )
 {
-
     std::ifstream ifs( aFile, std::ios::in | std::ios::binary );
 
     if( !ifs.is_open() )