Fix: "Freed pointer was not allocated error"

- close #356

- given that `SW_SoilWat->hist.file_prefix` is not initialized when `SW_SWC_construct()` is called (by SOILWAT2), then `!isnull(SW_SoilWat->hist.file_prefix)` was TRUE and we freed the not allocated pointer.

-> now, initialize instead of free the pointer

-> NOTE: rSOILWAT2 may now have a memory leak if it calls this function with a previously allocated `file_prefix` (e.g., if a `sw_exec()` fails and doesn't properly clean up).

NEWS.md

@@ -1,6 +1,9 @@
 # NEWS
 
 # SOILWAT2 v7.1.0-9000
+* Bugfixes
+    * Fix an error where a pointer was freed even though it was not allocated
+      (issue #356; @dschlaep).
 
 # SOILWAT2 v7.0.0
 * This version produces nearly identical simulation output

src/SW_SoilWater.c

@@ -572,11 +572,9 @@ void SW_SWC_construct(SW_SOILWAT* SW_SoilWat, LOG_INFO* LogInfo) {
 	/* =================================================== */
 	OutPeriod pd;
 
-	// Clear memory before setting it
-	if (!isnull(SW_SoilWat->hist.file_prefix)) {
-		Mem_Free(SW_SoilWat->hist.file_prefix);
-		SW_SoilWat->hist.file_prefix = NULL;
-	}
+	/* initialize pointer */
+	SW_SoilWat->hist.file_prefix = NULL;
+
 
 	// Clear the module structure:
 	memset(SW_SoilWat, 0, sizeof(SW_SOILWAT));