Prevent potential nullptr dereference.

Found by clang static analyzer.

libgnucash/engine/gncCustomer.c

@@ -987,10 +987,13 @@ gncCustomerGetCachedBalance (GncCustomer *cust)
 
 void gncCustomerSetCachedBalance (GncCustomer *cust, const gnc_numeric *new_bal)
 {
-    if (!new_bal && cust->balance)
+    if (!new_bal)
     {
-        g_free (cust->balance);
-        cust->balance = NULL;
+        if (cust->balance)
+        {
+            g_free (cust->balance);
+            cust->balance = NULL;
+        }
         return;
     }
 

libgnucash/engine/gncEmployee.c

@@ -962,10 +962,13 @@ gncEmployeeGetCachedBalance (GncEmployee *empl)
 
 void gncEmployeeSetCachedBalance (GncEmployee *empl, const gnc_numeric *new_bal)
 {
-    if (!new_bal && empl->balance)
+    if (!new_bal)
     {
-        g_free (empl->balance);
-        empl->balance = NULL;
+        if (empl->balance)
+        {
+            g_free (empl->balance);
+            empl->balance = NULL;
+        }
         return;
     }
 

libgnucash/engine/gncVendor.c

@@ -1040,10 +1040,13 @@ gncVendorGetCachedBalance (GncVendor *vend)
 
 void gncVendorSetCachedBalance (GncVendor *vend, const gnc_numeric *new_bal)
 {
-    if (!new_bal && vend->balance)
+    if (!new_bal)
     {
-        g_free (vend->balance);
-        vend->balance = NULL;
+        if (vend->balance)
+        {
+            g_free (vend->balance);
+            vend->balance = NULL;
+        }
         return;
     }