Fix bug in new PVStructure::getAs function

Fix seg fault when getAs is called with a string of the form x.y (or
x_1. ... x_n.y) where x or (x1. ... .x_i) is not a structure field.

Signed-off-by: Dave Hickin <david.hickin@diamond.ac.uk>

src/factory/PVStructure.cpp

@@ -170,7 +170,7 @@ PVField* PVStructure::GetAsImpl(const char *name) const
         if(*sep) {
             // this is not the requested leaf
             parent = dynamic_cast<PVStructure*>(child);
-            if(!child)
+            if(!parent)
                 throw std::runtime_error("mid-field is not a PVStructure"); //TODO: which sub field?
             child = NULL;
             name = sep+1; // skip past '.'