glsl: handle unsigned int wraparound in link_shaders()

v2: change check_explicit_uniform_locations() to return an
    unsigned 0 (Timothy Arceri)

We were storing the int result of check_explicit_uniform_locations()
in num_explicit_uniform_locs as an unsigned int which caused it to
be 4294967295 when a -1 was returned.

This in turn would cause the following error during linking:
error: count of uniform locations > MAX_UNIFORM_LOCATIONS(4294967295 > 98304)

Results from running piglit tests/all with this patch
and when ARB_explicit_uniform_location disabled:

changes:     178
fixes:       176
regressions: 2

The two regressions are for the following tests:
glean@glsl1-matrix column check (1)
glean@glsl1-matrix column check (2)
which regress from FAIL to CRASH.

The regressions are acceptable because the tests are currently failing due to
the aforementioned linker error.

Signed-off-by: Lars Hamre <chemecse@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/compiler/glsl/linker.cpp

@@ -3249,12 +3249,12 @@ reserve_subroutine_explicit_locations(struct gl_shader_program *prog,
  * any optimizations happen to handle also inactive uniforms and
  * inactive array elements that may get trimmed away.
  */
-static int
+static unsigned
 check_explicit_uniform_locations(struct gl_context *ctx,
                                  struct gl_shader_program *prog)
 {
    if (!ctx->Extensions.ARB_explicit_uniform_location)
-      return -1;
+      return 0;
 
    /* This map is used to detect if overlapping explicit locations
     * occur with the same uniform (from different stage) or a different one.
@@ -3263,7 +3263,7 @@ check_explicit_uniform_locations(struct gl_context *ctx,
 
    if (!uniform_map) {
       linker_error(prog, "Out of memory during linking.\n");
-      return -1;
+      return 0;
    }
 
    unsigned entries_total = 0;
@@ -3292,7 +3292,7 @@ check_explicit_uniform_locations(struct gl_context *ctx,
             }
             if (!ret) {
                delete uniform_map;
-               return -1;
+               return 0;
             }
          }
       }