Reset location when creating a method dispatcher

When creating a method dispatcher, the location of the underlying
method was copied. Under some circumstances, the name of the
location (loc.r) was already initialized, in which case the
method dispatcher shared a name with one of the methods, leading
to a C compiler error. By setting loc.r to nil when copying
the dispatcher information from the original method, we ensure
that the dispatcher C function gets its proper name.

compiler/cgmeth.nim

@@ -100,6 +100,7 @@ proc methodDef*(s: PSym, fromCache: bool) =
     if disp.typ.callConv == ccInline: disp.typ.callConv = ccDefault
     disp.ast = copyTree(s.ast)
     disp.ast.sons[bodyPos] = ast.emptyNode
+    disp.loc.r = nil
     if s.typ.sons[0] != nil: 
       disp.ast.sons[resultPos].sym = copySym(s.ast.sons[resultPos].sym)
     attachDispatcher(s, newSymNode(disp))