loader: check get_image_size() return value

since a negative value means it errored.

hw/core/loader.c:149:9: warning: Loss of sign in implicit conversion
    if (size > max_sz) {
        ^~~~
hw/core/loader.c:171:9: warning: Loss of sign in implicit conversion
    if (size > memory_region_size(mr)) {
        ^~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/core/loader.c

@@ -146,7 +146,7 @@ int load_image_targphys_as(const char *filename,
     int size;
 
     size = get_image_size(filename);
-    if (size > max_sz) {
+    if (size < 0 || size > max_sz) {
         return -1;
     }
     if (size > 0) {
@@ -168,7 +168,7 @@ int load_image_mr(const char *filename, MemoryRegion *mr)
 
     size = get_image_size(filename);
 
-    if (size > memory_region_size(mr)) {
+    if (size < 0 || size > memory_region_size(mr)) {
         return -1;
     }
     if (size > 0) {