Fix memory corruption in the aiGetExportFormatDescription() API. Clarify Exporter doc.

code/AssimpCExport.cpp

@@ -61,6 +61,8 @@ ASSIMP_API size_t aiGetExportFormatCount(void)
 // ------------------------------------------------------------------------------------------------
 ASSIMP_API const aiExportFormatDesc* aiGetExportFormatDescription( size_t pIndex)
 {
+	// Note: this is valid as the index always pertains to a builtin exporter,
+	// for which the returned structure is guaranteed to be of static storage duration.
 	return Exporter().GetExportFormatDescription(pIndex);
 }
 

code/Exporter.cpp

@@ -436,6 +436,11 @@ const aiExportFormatDesc* Exporter :: GetExportFormatDescription( size_t pIndex
 	if (pIndex >= GetExportFormatCount()) {
 		return NULL;
 	}
+	
+	// Return from static storage if the requested index is built-in.
+	if (pIndex < sizeof(gExporters) / sizeof(gExporters[0])) {
+		return &gExporters[pIndex].mDescription;
+	}
 
 	return &pimpl->mExporters[pIndex].mDescription;
 }