[streambuf.virt.put] Fix logic error in the specification of the overflow() effects, introduced by 0b640ed. (#1613)

Author: Eelis

source/iostreams.tex

@@ -3939,7 +3939,7 @@
 The pending sequence is defined as the concatenation of
 \begin{itemize}
 \item
-the empty sequence if \tcode{pbase()} is not null, otherwise the
+the empty sequence if \tcode{pbase()} is null, otherwise the
 \tcode{pptr() - pbase()}
 characters beginning at
 \tcode{pbase()}, followed by