Fix maybe-uninitialized warnings

Author: Flamefire

src_c/IMB_exchange.c

@@ -107,7 +107,7 @@ Output variables:
     int s_num = 0,
         r_num = 0;
     int s_tag, r_tag;
-    int left, right;
+    int left = 0, right = 0;
     MPI_Status  stat[2];
     MPI_Request request[2];
 

src_c/IMB_output.c

@@ -139,7 +139,7 @@ Input variables:
                       3 numbers (min/max/average)
 
 */
-    double scaled_time[MAX_TIME_ID];
+    double scaled_time[MAX_TIME_ID] = {0};
 
     int i, i_gr;
     int out_format;

src_cpp/helpers/original_benchmark.h

@@ -134,7 +134,7 @@ class OriginalBenchmark : public Benchmark {
             int np = item.np;
             int imod = *(item.extra_fields.as<int>());
             double t;
-            MPI_Datatype base_s_dt, base_r_dt, base_red_dt;
+            MPI_Datatype base_s_dt = 0, base_r_dt = 0, base_red_dt = 0;
             if (!initialized)
                 return;
             if (descr->stop_iterations)