Fix GCC 8.3.0 -Wmaybe-uninitialized on left_col

Fix the GCC 8.3.0 warnings that left_col and left_row may be used
uninitialized in the av1_nonrd_pick_inter_mode_sb function. Note that
those two variables are actually in the newmv_diff_bias function, which
is apparently inlined into the av1_nonrd_pick_inter_mode_sb function by
the compiler.

BUG=aomedia:3174

Change-Id: I2da7cd6e486e96ea2e34937ac23ed8df15a68d28

av1/common/mv.h

diff --git a/av1/common/mv.h b/av1/common/mv.h
index 3203bf7..0e4c405 100644
--- a/av1/common/mv.h
+++ b/av1/common/mv.h
@@ -23,6 +23,7 @@
 #endif
 
 #define INVALID_MV 0x80008000
+#define INVALID_MV_ROW_COL -32768
 #define GET_MV_RAWPEL(x) (((x) + 3 + ((x) >= 0)) >> 3)
 #define GET_MV_SUBPEL(x) ((x)*8)
 

av1/encoder/nonrd_pickmode.c

diff --git a/av1/encoder/nonrd_pickmode.c b/av1/encoder/nonrd_pickmode.c
index 5ee5761..af60883 100644
--- a/av1/encoder/nonrd_pickmode.c
+++ b/av1/encoder/nonrd_pickmode.c
@@ -1055,12 +1055,11 @@
   if (this_mode == NEWMV) {
     int al_mv_average_row;
     int al_mv_average_col;
-    int left_row, left_col;
     int row_diff, col_diff;
     int above_mv_valid = 0;
     int left_mv_valid = 0;
-    int above_row = 0;
-    int above_col = 0;
+    int above_row = INVALID_MV_ROW_COL, above_col = INVALID_MV_ROW_COL;
+    int left_row = INVALID_MV_ROW_COL, left_col = INVALID_MV_ROW_COL;
     if (bsize >= BLOCK_64X64 && content_state_sb.source_sad != kHighSad &&
         spatial_variance < 300 &&
         (mv_row > 16 || mv_row < -16 || mv_col > 16 || mv_col < -16)) {