patch 8.1.1207: some compilers give warning messages

Problem:    Some compilers give warning messages.
Solution:   Initialize variables, change printf() argument. (Christian
            Brabandt, closes #4305)
diff --git a/src/eval.c b/src/eval.c
index dd66287..bf82d02 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4051,7 +4051,7 @@
     varnumber_T	n1, n2;
 #ifdef FEAT_FLOAT
     int		use_float = FALSE;
-    float_T	f1 = 0, f2;
+    float_T	f1 = 0, f2 = 0;
 #endif
     int		error = FALSE;