formdata: ack read callback abort

When doing a multipart formpost with a read callback, and that callback
returns CURL_READFUNC_ABORT, that return code must be properly
propagated back and handled accordingly. Previously it would be handled
as a zero byte read which would cause a hang!

Added test case 587 to verify. It uses the lib554.c source code with a
small ifdef.

Reported by: Anton Bychkov
Bug: http://curl.haxx.se/mail/lib-2011-10/0097.html

tests/libtest/Makefile.inc

@@ -15,7 +15,7 @@ noinst_PROGRAMS = chkhostname \
   lib579 lib529 lib530 lib532 lib533 lib536 lib537 lib540 lib541 lib542	\
   lib543 lib544 lib545 lib547 lib548 lib549 lib552 lib553 lib554 lib555	\
   lib556 lib539 lib557 lib560 lib562 lib564 lib565 lib566 lib567 lib568	\
-  lib569 lib570 lib571 lib572 lib573 lib582 lib583 lib585
+  lib569 lib570 lib571 lib572 lib573 lib582 lib583 lib585 lib587
 
 chkhostname_SOURCES = chkhostname.c $(top_srcdir)/lib/curl_gethostname.c
 chkhostname_LDADD = @CURL_NETWORK_LIBS@
@@ -166,3 +166,6 @@ lib583_SOURCES = lib583.c $(SUPPORTFILES)
 
 lib585_SOURCES = lib500.c $(SUPPORTFILES)
 lib585_CPPFLAGS = $(AM_CPPFLAGS) -DLIB585
+
+lib587_SOURCES = lib554.c $(SUPPORTFILES)
+lib587_CPPFLAGS = $(AM_CPPFLAGS) -DLIB587

tests/libtest/lib554.c

@@ -42,6 +42,14 @@ static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
 {
   struct WriteThis *pooh = (struct WriteThis *)userp;
 
+#ifdef LIB587
+  (void)ptr;
+  (void)size;
+  (void)nmemb;
+  (void)userp;
+  return CURL_READFUNC_ABORT;
+#else
+
   if(size*nmemb < 1)
     return 0;
 
@@ -53,6 +61,7 @@ static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
   }
 
   return 0;                         /* no more data left to deliver */
+#endif
 }
 
 int test(char *URL)