Avoid uninitialized variable warning in entropy_gather_internal

The variable ret was always initialized in entropy_gather_internal,
but `gcc -Werror=maybe-uninitialized` rightfully complained that it
was unable to determine this statically. Therefore, tweak the
problematic case (ctx->source_count == 0) to not use ret in that case.
diff --git a/library/entropy.c b/library/entropy.c
index 5a8321b..518d982 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -196,13 +196,11 @@
  */
 static int entropy_gather_internal( entropy_context *ctx )
 {
-    int ret, i;
+    int ret = POLARSSL_ERR_ENTROPY_NO_SOURCES_DEFINED;
+    int i;
     unsigned char buf[ENTROPY_MAX_GATHER];
     size_t olen;
 
-    if( ctx->source_count == 0 )
-        return( POLARSSL_ERR_ENTROPY_NO_SOURCES_DEFINED );
-
     /*
      * Run through our entropy sources
      */