Fix #252: gd_error() garbles variable arguments

Currently gd_error() forwards to gd_error_ex(). However, both functions
accept a variable number of arguments, and simply forwarding the va_list
isn't portable, see <http://c-faq.com/varargs/handoff.html>. This article
also describes the usual workaround, namely to let the second function
accept a va_list instead of variable number of arguments.

We do so by introducing a static helper, what does not affect API/ABI
compatibility.

src/gd.c

@@ -96,12 +96,19 @@ void gd_stderr_error(int priority, const char *format, va_list args)
 
 static gdErrorMethod gd_error_method = gd_stderr_error;
 
+static void _gd_error_ex(int priority, const char *format, va_list args)
+{
+	if (gd_error_method) {
+		gd_error_method(priority, format, args);
+	}
+}
+
 void gd_error(const char *format, ...)
 {
 	va_list args;
 
 	va_start(args, format);
-	gd_error_ex(GD_WARNING, format, args);
+	_gd_error_ex(GD_WARNING, format, args);
 	va_end(args);
 }
 void gd_error_ex(int priority, const char *format, ...)
@@ -109,9 +116,7 @@ void gd_error_ex(int priority, const char *format, ...)
 	va_list args;
 
 	va_start(args, format);
-	if (gd_error_method) {
-		gd_error_method(priority, format, args);
-	}
+	_gd_error_ex(priority, format, args);
 	va_end(args);
 }
 

tests/CMakeLists.txt

@@ -21,6 +21,7 @@ if (BUILD_TEST)
 		bmp
 		freetype
 		gd
+		gd_error
 		gd2
 		gdimagearc
 		gdimagecolorclosest

tests/Makefile.am

@@ -15,6 +15,7 @@ TESTS =
 include bmp/Makemodule.am
 include freetype/Makemodule.am
 include gd/Makemodule.am
+include gd_error/Makemodule.am
 include gd2/Makemodule.am
 include gdimagearc/Makemodule.am
 include gdimagecolorclosest/Makemodule.am

tests/gd_error/.gitignore

@@ -0,0 +1 @@
+/bug00252

tests/gd_error/CMakeLists.txt

@@ -0,0 +1,5 @@
+LIST(APPEND TESTS_FILES
+	bug00252
+)
+
+ADD_GD_TESTS()

tests/gd_error/Makemodule.am

@@ -0,0 +1,5 @@
+libgd_test_programs += \
+	gd_error/bug00252
+
+EXTRA_DIST += \
+	gd_error/CMakeLists.txt

tests/gd_error/bug00252.c

@@ -0,0 +1,24 @@
+/* See <https://github.com/libgd/libgd/issues/252>. */
+
+#include "gd.h"
+#include "gd_errors.h"
+#include "gdtest.h"
+
+void my_error_handler(int priority, const char *format, va_list args)
+{
+    int n;
+
+    /* The following works according to
+       <https://www.gnu.org/software/libc/manual/html_node/Receiving-Arguments.html>.
+       It might not be portable, though. */
+	n = va_arg(args, int);
+    gdTestAssert(n == 42);
+}
+
+int main()
+{
+    gdSetErrorMethod(my_error_handler);
+    gd_error_ex(GD_WARNING, "The answer is %i\n", 42);
+    gd_error("The answer is %i\n", 42);
+    return gdNumFailures();
+}